glossary: single-source glossary entries

[PLeVasseur]

Summary

• Add glossary-entry/glossary-include directives and a generator so chapter + glossary definitions share one source of truth.
• Integrate glossary generation into make.py/CI and add HTML diff verification for parity checks.
• Migrate glossary content to the new directive format and document the workflow.

Closes #655

Guiding tenets

• The generated build/generated.glossary.rst must be identical to today's glossary.rst, byte-for-byte
• We would still commit glossary.rst to have a stable record in the repo
  • The glossary generation and commit would occur on changes to things which warranted a glossary change
  • This will be automated
• There is a verification script I'm running during this migration
  • generated build/html must be identical, byte-for-byte
  • generated paragraph-ids.json must be identical, once accounting for ordering (as I found the build process is not deterministic in how this is generated)
• Migrate 20 terms at a time, sanity check them, then proceed
• Perform a final set of holistic checks

Reference alignment

• Not applicable (workflow/tooling change).

Testing

• Some careful checks and tests were done incrementally along the way to ensure that the build/generated.glossary.rst is identical to the src/glossary.rst.

Examples

Before (glossary-only entry in src/glossary.rst.inc):

.. _fls_f3ferow5ugp:

associated item
^^^^^^^^^^^^^^^

:dp:`fls_o5ysjk7l91ni`
An :dt:`associated item` is an :t:`item` that appears within an
:t:`implementation` or a :t:`trait`.

:dp:`fls_44vtqu7tvhi2`
See :s:`AssociatedItem`.

After (single-source entry in src/associated-items.rst):

.. glossary-entry:: associated item
   :glossary-dp: fls_f3ferow5ugp

   :glossary:
     :dp:`fls_o5ysjk7l91ni`
     An :dt:`associated item` is an :t:`item` that appears within an
     :t:`implementation` or a :t:`trait`.

     :dp:`fls_44vtqu7tvhi2`
     See :s:`AssociatedItem`.
   :chapter:
     :dp:`fls_ckzd25qd213t`
     An :t:`associated item` is an :t:`item` that appears within an
     :t:`implementation` or a :t:`trait`.

Before (glossary-only entry in src/glossary.rst.inc):

.. _fls_j44ow2k5va3s:

incomplete associated constant
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

:dp:`fls_bq48gl84bul0`
An :dt:`incomplete associated constant` is an :t:`associated constant` without
a :t:`constant initializer`.

After (glossary-only entry placed near related chapter content in
src/associated-items.rst):

.. glossary-entry:: incomplete associated constant
   :glossary-dp: fls_j44ow2k5va3s

   :glossary:
     :dp:`fls_bq48gl84bul0`
     An :dt:`incomplete associated constant` is an :t:`associated constant` without
     a :t:`constant initializer`.