docs: schema docs for Protobuf with protoc-gen-doc and Avro #9824

Where

• tools/docs/support_matrix/languages.bzl:130-166: DOC_TOOLS is None for protobuf and avro, so the API docs column says no.
• dx docs has no shard for it: shards live in tools/docs/<lang>/defs.bzl (for example tools/docs/jvm/defs.bzl:101), are selected by tools/docs/aspect.bzl, and are listed in docs/cli/commands/docs.md:84.

Fix

• Protobuf: run protoc-gen-doc through the pinned protoc. Avro: pick a maintained schema doc generator; if none exists, set the cell to n/a with a reason instead.
• Add <lang>_doc_shard in a new tools/docs/<lang>/defs.bzl, wire it into tools/docs/aspect.bzl so dx docs finds the library with no setup, and add a fixture like tools/docs/jvm/fixture/BUILD.bazel.
• Pin the tool hermetically on every platform the language supports. If a platform cannot run it, record the reason in the support record.
• Set the DOC_TOOLS entry, add the row to docs/cli/commands/docs.md and the language README, and run bazel run //tools/docs/support_matrix:update.

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https://claude.ai/code/session_01Xut2hwESun1HL2eSVd1d5B