Sanitize Google descriptions so ExDoc docs build clean

[mhsdef]

What

mix hex.publish emitted ExDoc warnings while generating docs for googly_document_ai. Two kinds, both from Google's discovery descriptions being emitted verbatim into @moduledoc/@doc:

1. "Closing unclosed backquotes" — a description opens an inline code span it never closes (Format is `projects/.../{schema_version}). EarmarkParser counts backticks across the whole moduledoc, finds an odd total, and reports it at end-of-input — which is why the warning fingered the wrong field (type instead of active_schema_version).
2. "references file ... does not exist" — [regional and multi-regional support](/document-ai/docs/regions); ExDoc resolves any /… target as a local file.

How

Both are fixed in Renderer.doc/3 — the single place every description flows through:

• normalize_markdown_links/2 rewrites root-relative ](/foo) targets to absolute URLs via URI.merge/2, resolving against the API's own documentation host (documentationLink), threaded in as docs_link(token). This matters because googly targets any Google API and hosts differ — Cloud Storage lives under developers.google.com/storage/docs/json_api/, while Vision/DocumentAI are under cloud.google.com. Absolute and protocol-relative (//host) targets are left alone.
• close_dangling_inline_code/1 appends a closing backtick when a description's backtick count is odd. This is deliberately a small upstream-prose repair, not real Markdown parsing — pulling in EarmarkParser just to fix Google's typos isn't worth it, and the generated client runtime stays req + jason only.

docs_link/1 is now public and shares a @default_docs_link fallback with doc/3.

Testing

• Added 5 doc/3 unit tests (unterminated code span; balanced backticks untouched; root-relative link against the default host and against an explicit developers.google.com base; absolute/protocol-relative untouched). Full suite: 65 passed.
• Regenerated all clients — output is byte-identical to before (DocumentAI's host is cloud.google.com), so no committed client changed beyond the original one-line link fix.
• cd clients/googly_document_ai && mix docs → zero warnings; mix compile --force --warnings-as-errors → clean (340 files).

Notes

Per the golden rule, the fix lives in the generator/templates (never hand-edited generated code); the 4 regenerated DocumentAI model files are committed output.