Refactored docs (#983)

Refactoring existing docs into four new areas: tutorials, how-tos,
experiment guides, and the API reference. With contributions from
@chriseclectic, @wshanks, @jaleipekoglu, and @spencerking.

Main changes:
- Added introduction tutorials
- Added how-tos (this has a lot of room for future expansion)
- Added cards linking to each documentation area on the index page
- Organized experiment guides by type and with representative thumbnails
- Moved detailed API docs such as Curve Analysis to tutorials
- Consolidated previous calibrations guides into the new calibrations
tutorial
- Changed the `See Also` section in docstrings to render the rst
directly instead of parsing modules
- Closes #1014 by replacing `sphinx-panels` with `sphinx-design`
- Upgraded `qiskit-sphinx-theme` to >=1.10
- Removed usage of `qiskit-ibmq-provider` in docs and linked to
migration guide
- Parallelized sphinx build run (CI docs build went from 13 minutes to
10:20)
- Various fixes to remove all Sphinx build warnings and forces CI to
fail if there's a warning (warnings might still exist on mac)
- Cleaned up and removed unnecessary style and template files
- Added commands `tox -edocs-parallel` for a parallel build and renamed
no jupyter-execute command to `tox -edocs-minimal`

---------

Co-authored-by: Will Shanks <willshanks@us.ibm.com>
Co-authored-by: Will Shanks <wshaos@posteo.net>

.github/ISSUE_TEMPLATE/FEATURE_REQUEST.md

@@ -10,4 +10,4 @@ assignees: ''
 <!-- ⚠️ If you do not respect this template, your issue will be closed -->
 <!-- ⚠️ Make sure to browse the opened and closed issues to confirm this idea does not exist. -->
 
-### What is the expected behavior?
+### Suggested feature

.github/ISSUE_TEMPLATE/NEW_EXPERIMENT.md

@@ -17,7 +17,7 @@ your proposal. -->
 ## General details
 
 ### Experiment name
-<!-- What is the experiment class name? This name will also be used in the API documentation and tutorial. -->
+<!-- What is the experiment class name? This name will also be used in the documentation. -->
 
 ### Experiment type
 <!-- What is the experiment type? Characterization, calibration, verification, validation, or other? -->
@@ -86,7 +86,7 @@ experiment data is displayed correctly in the results DB webpage
 - [ ] Add unit testing for the experiment and analysis classes. If needed implement a mock-backend for your experiment
 Include in your testing running the experiment in the context of `ParallelExperiment`
 - [ ] Write API docs for all your API methods. Follow the guideline [here](https://github.com/Qiskit/qiskit-experiments/blob/main/CONTRIBUTING.md)
-- [ ] Write a tutorial for your experiment. Follow the guideline [here](https://github.com/Qiskit/qiskit-experiments/blob/main/docs/tutorials/GUIDELINES.md)
+- [ ] Write a user guide for your experiment. Follow the guideline [here](https://github.com/Qiskit/qiskit-experiments/blob/main/docs/GUIDELINES.md)
 - [ ] Add a new release note. Follow the guideline [here](https://github.com/Qiskit/qiskit-experiments/blob/main/CONTRIBUTING.md#adding-a-new-release-note) 
 - [ ] Ask for a final review for the implementation, documentation and testing
 - [ ] Celebrate!

.github/workflows/main.yml

@@ -97,7 +97,7 @@ jobs:
           python -m pip install -U tox
           sudo apt-get install -y pandoc graphviz
       - name: Build Docs
-        run: tox -edocs
+        run: tox -edocs-parallel -- -W
       - name: Compress Artifacts
         run: |
           mkdir artifacts