Add React front end guide to published docs

[brainlid]

Problem

docs/using_with_a_react_front_end.md shipped in v0.11.1 but was never added to extras(), so it does not render on HexDocs. It is in the hex package (the docs directory is in package/files) but has no browsable page.

The groups_for_extras: [Docs: Path.wildcard("docs/*.md")] config looks like it would pick the file up, but that wildcard only assigns sidebar groups to extras already in the list. It does not discover pages.

Solution

Add the guide to extras(), placed after docs/subscriptions_and_presence.md since the guide names that doc as prerequisite reading and within-group sidebar order follows list order.

Changes

• mix.exs: add docs/using_with_a_react_front_end.md to extras()

Testing

Ran mix docs locally and confirmed:

• using_with_a_react_front_end.html generates
• The prerequisite link rewrites from .md to .html, so it points at the real page
• Sidebar order is subscriptions_and_presence then using_with_a_react_front_end then middleware
• All 6 mermaid blocks render as <code class="mermaid">, matching the selector in before_closing_body_tag

mix format --check-formatted passes.