Upstream caffeinelabs/motoko PR #6132 transformed doc/md/ into a
Starlight-ready state: clean filenames, sidebar.order frontmatter,
aside syntax normalized, mo:base→mo:core rewrites, numeric prefixes
removed, and index.md stubs added for subdirectory groups. This PR
adopts those changes in the developer-docs sync pipeline.

Changes:
- scripts/sync-motoko.sh: rewrite as plain rsync (~35 lines vs ~180).
  All the complex bash transforms (frontmatter injection, .mdx→.md
  conversion, numeric prefix stripping, slug collision guards) are
  no-ops now that upstream handles them.
- scripts/postprocess-motoko.mjs: remove seven steps that upstream
  now handles. Critical fix: the old script explicitly deleted the
  new index.md stubs PR #6132 added, breaking autogenerate. Also
  adds a general docs.internetcomputer.org/ prefix rule to handle
  links that upstream PR #6132 §5 rewrote from the retired portal
  format to the current docs.internetcomputer.org/ domain.
- sidebar-motoko.mjs: replace 130-line explicit page list with a
  section-level autogenerate config. Uses explicit section labels
  ("ICP features") to avoid the "Icp Features" label that plain
  autogenerate would produce for the icp-features/ directory.
- ec.config.mjs + syntaxes/bnf.tmLanguage.json: register BNF grammar
  for Shiki so the 30 BNF blocks in language-manual.md no longer
  emit language-not-found warnings.
- .docs-plan/motoko-repo-sync-proposal.md: delete (resolved by PR
  #6132; tracked in issue #265).
- .sources/motoko: bump to a9cc50f (master after PR #6132 merge;
  no release tag cut yet — this is a pre-release test pin).
- docs/languages/motoko/**: full resync from upstream (77 content
  files updated + 6 new index.md stubs for autogenerate groups).

Build: 208 pages, no errors.

The new autogenerate approach in sidebar-motoko.mjs broke the Motoko
sidebar in two ways:

1. Group labels showed raw directory names ("actors", "basic-syntax")
   instead of "Actors", "Basic syntax". In Starlight v0.38, groupFromDir
   uses the raw directory key as the label — it does not read index.md
   frontmatter to derive the label.

2. The index.md stubs (added by upstream PR #6132) appeared as duplicate
   page entries within the autogenerate groups ("Actors" page inside the
   "actors" group), and the root "Motoko" page showed as "Motoko" instead
   of "Overview".

Fix:
- sidebar-motoko.mjs: restore explicit items for fundamentals/ (matches
  the live site structure) and reference/ (needed for "Motoko changelog"
  label). icp-features/ keeps autogenerate — it is a flat list of pages
  with correct sidebar.order values and no subdirectories.
- postprocess-motoko.mjs: add sidebar.hidden: true to all subdirectory
  index.md stubs. This prevents them from appearing as stray page entries
  in autogenerate results. The root index.md (which has content) is
  excluded from this rule.

sidebar.mjs will exist in doc/md/ once caffeinelabs/motoko#6139 merges.
postprocess-motoko.mjs reads it directly from the submodule path; it
should not be copied into docs/languages/motoko/ as a stray non-content
file.

The upstream changelog.md stub uses a build-time file include
(```md file=<motokoRoot>/Changelog.md```) that works locally but
produces an empty page in the icp deploy CI recipe (the submodule is
not in scope when the asset canister recipe runs npm run build).

Fix: expand the include in postprocess-motoko.mjs before walkAndProcess
so the file is fully self-contained in docs/ and em-dash replacement
and link rewrites apply to the inlined content too.