Create specification for website manifest

[lidel]

This is a placeholder to create RFC proposal that improves the experience of publishing websites on IPFS.

• configuration should live in the content root directory
• must work on every gateway, so support for "manifest" needs to be implemented in go-ipfs and js-ipfs
• need to create and agree on spec first

The bare minimum, we need ways to specify some behaviors under the content root (CID used for Origin isolation):

• redirects (relative, absolute?)
• content-type
  • where are use cases where people can't use IPFS because their tooling expect specific content-type
  • this should be possible
    • per path (specific file or every file under a directory)
    • per extension
• some safety-related HTTP headers (eg. [X-Frame-Options](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-- Frame-Options) to disable embedding via iframe or Content-Security-Policy)

Related discussions:

• https://github.com/ipfs/go-ipfs/issues/7392
• https://github.com/ipfs/go-ipfs/issues/6214