Docs: change theme and overhaul structure (#689)

The documentation is currently quite outdated, and the main page was
quite an overwhelming amount of information, with very little guidance
on where to begin.

Here we switch to the `sphinx-book-theme` used by several other AiiDA
projects and clean up the layout somewhat. We also add:

* The CLI reference
* An installation section on the main page
* A Getting started section on the main page that points to the AiiDA
Quantum ESPRESSO tutorial on the AiiDA tutorials readthedocs.
* A warning on the main page that indicates that some of the material
may be outdated.
* Add an amazingly beautiful logo that will never require any further 
revisions, ever.

.pre-commit-config.yaml

@@ -42,6 +42,7 @@ repos:
         name: yapf
         types: [python]
         args: ['-i']
+        exclude: 'docs/source/conf.py'
 
 -   repo: https://github.com/PyCQA/pylint
     rev: pylint-2.6.0

docs/source/_static/aiida-custom.css

@@ -0,0 +1,117 @@
+/* Fix CSS of top bar link icons */
+a.nav-link.nav-external i {
+    padding-left: 0.3em !important;
+    font-size: inherit !important;
+    vertical-align: inherit !important;
+}
+/* Current fix for https://github.com/pandas-dev/pydata-sphinx-theme/issues/193 */
+dl.field-list {
+    display: grid;
+    grid-template-columns: fit-content(30%) minmax(0, 1fr);
+}
+/* For icon unicodes see https://fontawesome.com/v4.7.0/icons/  */
+.title-icon-rocket .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f135"
+}
+.title-icon-info-circle .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f05a"
+}
+.title-icon-question-circle .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f059"
+}
+.title-icon-download .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f019"
+}
+.title-icon-external-link .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f08e"
+}
+.title-icon-lightbulb .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f0eb"
+}
+.title-icon-wrench .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f0ad"
+}
+.title-icon-cog .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f013"
+}
+.title-icon-cogs .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f085"
+}
+.title-icon-code-fork .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f126"
+}
+/* Semantic icon names */
+.title-icon-launch-software .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f135" /* rocket */
+}
+.title-icon-install-software .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f019" /* download */
+}
+.title-icon-information .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f05a" /* info-circle */
+}
+.title-icon-tip .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f0eb" /* lightbulb */
+}
+.title-icon-important .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f06a" /* exclamation-circle */
+}
+.title-icon-warning .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f071" /* exclamation-triangle */
+}
+.title-icon-troubleshoot .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f0ad"  /* wrench */
+}
+.title-icon-read-more .admonition-title:before {
+    margin-right:.5rem;
+    content: "\f518"  /* external-link */
+}
+
+.dropdown-group .dropdown .summary-title {
+    border-bottom: 0 !important;
+    font-weight:700 !important;
+}
+.dropdown-group .dropdown:not(:last-child) {
+    margin-bottom: 0 !important;
+    border-radius: 0 !important;
+}
+.dropdown-group .dropdown:first-child,
+.dropdown-group .dropdown:first-child .summary-title {
+    border-radius: 1rem 1rem 0rem 0rem !important;
+}
+.dropdown-group .dropdown:last-child,
+.dropdown-group .dropdown:last-child .summary-title {
+    border-radius: 0rem 0rem 1rem 1rem !important;
+}
+
+.dropdown-group .dropdown:last-child {
+    margin-bottom: 24px !important;
+}
+
+div.admonition :last-child {
+    margin-bottom: 0
+}
+
+div.highlight-bash div.highlight {
+    background-color: aliceblue;
+}
+div.highlight-console div.highlight {
+    background-color: aliceblue;
+}

docs/source/_static/aiida-qe-custom.css

@@ -0,0 +1,28 @@
+.logo-table td {
+    padding: 20px 10px;
+}
+
+.center {
+    text-align: center;
+    max-width: 90%;
+}
+
+.bigfont {
+    font-size: 140%;
+}
+
+div.navbar-brand-box a.navbar-brand img {
+    display: block;
+    height: auto;
+    width: auto;
+    max-height: 100vh;
+    max-width: 90%;
+    margin: 0 auto;
+    padding-left: 30px
+}
+
+@media (min-width: 768px) {
+    div.navbar-brand-box a.navbar-brand img {
+        max-height: 100vh !important
+    }
+}

docs/source/cli/calculation.rst

@@ -0,0 +1,3 @@
+.. click:: aiida_quantumespresso.cli.calculations:cmd_calculation
+    :prog: aiida-quantumespresso calculation
+    :nested: full

docs/source/cli/data.rst

@@ -0,0 +1,3 @@
+.. click:: aiida_quantumespresso.cli.data:cmd_data
+    :prog: aiida-quantumespresso data
+    :nested: full

docs/source/cli/index.rst

@@ -0,0 +1,11 @@
+
+#############
+CLI reference
+#############
+
+.. toctree::
+    :maxdepth: 2
+
+    calculation <calculation>
+    data <data>
+    workflow <workflow>

docs/source/cli/workflow.rst

@@ -0,0 +1,3 @@
+.. click:: aiida_quantumespresso.cli.workflows:cmd_workflow
+    :prog: aiida-quantumespresso workflow
+    :nested: full