Move `librehost.json` under `/.well-known/`

We started off with putting the librehost.json file under the domain's web root. Recently privacytools.io created a JSON file under /.well-known/librehost.json. The /.well-known/ directory is indeed made to host that kind of data (cf. RFC8615).

Using a location under /.well-known/ requires a formal registration with IANA (see Well Known URIs Registry) in the form of an RFC at IETF. Therefore this move should come as a 'natural' evolution of our discovery protocol. Yet, it requires some effort in order to obtain a permanent identifier from IANA.

Discussion needs to happen over these requirements:

• set a proper location, e.g.: /.well-known/librehosters/librehost.json and register librehosters with IANA
  (note that we could register /.well-known/librehost.json, but setting a directory enables future extensions, e.g., supporting such things as /.well-known/librehosters/services.json)
• follow registration instructions in sections 3 and 3.1 to write an RFC
  • open relevant shared draft topic on talk.libreho.st
  • write, discuss, and validate RFC (see for example https://tools.ietf.org/html/rfc7033#section-10)
  • update all current librehost.json to new format and location
  • publish RFC