Improve rspc docs

[oscartbeaumont]

• Feedback for “Vanilla Client” rspc#318
• website consuming much cpu rspc#302
• Docs around Next.js caching rspc#280
• Remove framework selector from docs rspc#262
• Example of using anyhow rspc#215
• discuss transport protocol
• discuss building abstractions on rspc
• integrations & middleware sections
• Proxy and queryOptions syntax
• Show the default setup for HTTP adapter
• Show a custom endpoint accepting other formats (FormData, json, etc)
• In general the approach is that rspc doesn't support content-types out of the box, that's on you.
• Uploading multiple files in one request will cause them to block. We accept this.