site: Preserve heading anchor IDs in docs2mdx conversion (https://github.com/bazelbuild/bazel/pull/30704)

## Summary

Reference doc pages like [common-definitions](https://bazel.build/versions/9.1.0/reference/be/common-definitions) expose a Contents section with links such as `#common-attributes`, but those anchors are broken on the published site.

The source Velocity template `common-definitions.vm` renders headings with explicit ids:

```html
<h2 id='common-attributes'>Attributes common to all build rules</h2>
```

After `docs2mdx.py` converts the generated HTML to MDX, the heading becomes plain markdown with no anchor:

```markdown
## Attributes common to all build rules
```

This PR updates `docs2mdx.py` to convert HTML headings with `id` attributes to MDX anchor syntax (`## Title {#id}`) before markdownify runs, and restores anchors after markdownify escapes curly braces in heading text.

Fixes bazelbuild/bazel#30617

## Test plan

### Unit tests
- [x] `bazel test //scripts/docs:rewriter_test`
- [x] `bazel test //scripts/docs:docs2mdx_test` — single/double-quoted ids, extra attributes, h3 headings

### Mintlify preview
Preview: https://bazel-pr-30704.mintlify.app/

| Check | URL | Expected |
|-------|-----|----------|
| [ ] Anchor resolves | `/reference/be/common-definitions#common-attributes` | Page scrolls to "Attributes common to all build rules" |
| [ ] Contents TOC link | `/reference/be/common-definitions` → click `#common-attributes` in Contents | Same section |
| [ ] Other anchors | `#typical-attributes`, `#common-attributes-tests` | Resolve correctly |

- [x] Preview deployed (bazel-docs bot comment)
- [ ] Anchor deep links verified in preview (requires reference doc regen in preview build)

### Post-merge
- [ ] Regenerate reference docs via `bazel build --config=docs //src/main/java/com/google/devtools/build/lib:gen_mdx_reference_docs`

Closes #30704.

PiperOrigin-RevId: 972587453
Change-Id: Ibfef830fce4525d746aff992bd990bab3f501eac