fix: fix MDX syntax errors in 11 docs pages (#251)

[alan707]

Summary

Fixes MDX syntax errors in 11 docs pages so Mintlify can parse and render them, removing them from .mintignore.

Files fixed

File	Issues fixed
community/users.mdx	{: .external} link class
concepts/build-files.mdx	{: .external}, devsite nav table
concepts/dependencies.mdx	{ bare braces in <pre>, import at col 0, inline <pre>, devsite nav table
concepts/labels.mdx	{{ "<var>" }} template → backtick, devsite nav table
concepts/visibility.mdx	{#...} GitHub heading anchor
contribute/search.mdx	{:#...} devsite anchors, <br> → <br />, \</td> → &#92;</td>
docs/cc-toolchain-config-reference.mdx	{:#...} anchors, <col> → self-closing, inline <pre>, multi-line <td>, 4-space code block with <inc0>
docs/user-manual.mdx	175 {:#...} anchors, {{ }} templates, ${} shell var escaping, inline <pre>, stray </p>
extending/config.mdx	{:#...} anchors, bare {String:Object} dict literal
extending/platforms.mdx	{:#...} anchors, {: .external}

Also adds upstream/** to .mintignore to exclude the git submodule from mint validate scans.

Remaining in .mintignore

• Generated reference docs (reference/be/**, rules/lib/**) — need docs2mdx.py fix upstream
• Files covered by [Docs Preview] Bazel Docs: Remove all DevSite tags from new .mdx docs. bazelbuild/bazel#28920 (query/language.mdx, reference/glossary.mdx, reference/test-encyclopedia.mdx, reference/flag-cheatsheet.mdx, fwe@ files) — will be addressed after that PR merges
• Versioned copies (*/community/roadmaps-build-api.mdx, */external/advanced.mdx, */query/cquery.mdx)

Test plan

• mint validate passes with zero parsing errors locally
• Mintlify preview deployment builds successfully
• Previously-broken pages render correctly in preview

Partial fix for #251