fix(docs): drop the inert bare /mcp redirect

[a-essawy]

Follow-up to #52, verified against production after that deploy landed.

/docs/mcp is Mintlify's own hosted MCP server endpoint for this docs site, not a page. It is POST-only JSON-RPC, so a GET correctly returns 405, and the platform route takes precedence over the redirects table.

$ curl -X POST https://rendobar.com/docs/mcp -d '{"jsonrpc":"2.0","id":1,"method":"initialize",...}'
event: message
data: {"result":{"protocolVersion":"2024-11-05","capabilities":{"tools":...},
       "serverInfo":{"name":"Rendobar","version":"1.0.0"},
       "instructions":"This Model Context Protocol server provides search and
       retrieval tools for the Rendobar site..."}}

So the /mcp -> /mcp-server rule #52 added never fires. It was written to cover a 405 that turns out not to be a defect. Removing it, because leaving it tells the next person reading this file that /docs/mcp redirects when it does not, and validate-nav waves it through (the destination is a real page, so the check cannot see that the source is shadowed).

The real fix from #52 is untouched and still live:

/docs/mcp/overview       308 -> /docs/mcp-server  200
/docs/mcp/installation   308 -> /docs/mcp-server  200

Test plan

$ npm run validate:nav          # Checked 16 pages, 16 nav entries, 22 redirects. All consistent.
$ npm run validate:frontmatter  # Checked 16 MDX files. All frontmatter valid.

No behaviour change to verify after deploy: the rule being removed has no observable effect today.