Automate the manually maintained proposals index file to avoid merge conflicts

[smoparth]

Replace the hand-maintained file list in docs/proposals/index.rst with Sphinx's :glob: directive to eliminate recurring merge conflicts when multiple proposal PRs are open simultaneously.

Context

Every PR that adds a new proposal document must also add a line to docs/proposals/index.rst. When two or more proposal PRs are open at the same time, they both modify the same small block of lines, and the second one to merge always has a merge conflict(e.g. #1084).

Proposed Change

Replace the explicit file list with a glob pattern:

Fromager Enhancement Proposals
==============================

.. toctree::
   :maxdepth: 1
   :glob:

   *

The :glob: directive automatically discovers all .rst and .md files in the directory (excluding index.rst itself) and sorts them alphabetically.

Acceptance Criteria

• docs/proposals/index.rst uses :glob: directive instead of explicit file list
• All existing proposals still appear in the rendered docs
• A new proposal can be added by creating the file alone, without modifying index.rst