docs: make the hosted instance the front door, move self-hosting out

The README asked a newcomer to choose between three setups before it told them
what the thing did. Now there is one obvious path — paste
https://gsc.k-o.pro/mcp into Claude and sign in with Google — and the URL is in
the first screenful, above the fold, in a copyable block.

Deployment moved to docs/self-hosting.md rather than being deleted: this is an
Apache-2.0 repo, so the code is the self-hosting instructions whether or not
they are written down, and someone who wants their users' Google tokens on
their own hardware should not have to reverse-engineer the compose file. The
README links to it once, from a subsection that says plainly that the hosted
instance is the same code.

Local stdio install is kept, and now says why anyone would choose it over the
hosted service — the Google token never leaves your machine, and it is the only
mode with the write tools, which the hosted service deliberately does not
offer. That is a real reason, not a consolation.

New "While it is in beta" section, because the alternative was a README that
tells the world to sign in to an app Google caps at 100 approved accounts.
It names access_denied as the symptom and points at the issue tracker, so the
cap reads as a queue rather than a bug.

Also: a "Leaving" section directly under the connect instructions, since a
service asking for your Google account should say how to revoke it before it
asks; troubleshooting split into hosted-user symptoms and operator symptoms;
Configuration reduced to what a local run needs; and package homepage plus the
GitHub homepage field point at the service.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: kodotpro