docs: add ADR for Python-free static asset pipeline

docs/decisions/0016-build-assets-without-python.rst

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+Building static assets without Python
+#####################################
+
+Status
+******
+
+Pending
+
+Will be moved to *Accepted* upon completion of re-implementation.
+
+Context
+*******
+
+edx-platform assets (including legacy LMS views, legacy CMS views, and built-in XBlock views) are built using Python functions in the ``pavelib/`` directory. The build process includes:
+
+* Copying files from ``node_modules/`` into various "vendor" directories to support RequireJS frontends.
+* Collecting assets from XModule-style XBlocks that are built-in to the platform.
+* Compiling common SCSS into CSS, both for base files and for theme-provided files.
+* Running Webpack.
+* Collecting assets into the ``STATIC_ROOT``.
+
+Tutor invokes these functions via its custom `openedx-assets`_ Python script. Devstack and the old Ansible installation both invoke these functions via the ``paver update_assets`` command.
+
+.. _openedx-assets: https://github.com/overhangio/tutor/blob/open-release/olive.1/tutor/templates/build/openedx/bin/openedx-assets.
+
+Updating the asset build pipeline will be necessary for several current and upcoming efforts, including:
+
+* `Finish upgrading frontend frameworks <https://github.com/openedx/edx-platform/issues/31616>`_
+* `Move node_modules outside of edx-platform in Tutor's openedx image <https://github.com/openedx/wg-developer-experience/issues/150>`_
+* `Move static assets outside of edx-platform in Tutor's openedx image <https://github.com/openedx/wg-developer-experience/issues/151>`_
+
+This has caused us to consider the value of updating the asset pipeline in place, versus rewriting and simplying it first.
+
+Decision
+********
+
+TODO
+
+Rationale:
+
+    * Other parts of pavelib have already been reimplemented, like Python
+      unit tests. We're following that trend.
+    * The Python logic in pavelib is harder to understand than simple
+      shell scripts.
+    * pavelib has dependencies (Python, paver, edx-platform, other libs)
+      which means that any pavelib scripts must be executed later in
+      the edx-platform build process than we might want them to. For
+      example, in a Dockerfile, it might be more performant to process
+      npm assets *before* installing Python, but as long as we are still
+      using pavelib, that is not an option.
+    * The benefits of paver have been eclipsed by other tools, like
+      Docker (for requisite management) and Click (for CLI building).
+    * In the next couple commits, we make improvements to
+      process-npm-assets.sh. These improvements would have been possible
+      in the pavelib implementation, but would have been more complicated.
+...
+
+Consequences
+************
+
+TODO
+
+...
+
+Alternatives Considered
+***********************
+
+TODO
+
+...
+