Allow *.md generation for any chain (w/ retrieved info)

[jacogr]

Basic support for extended (not Substrate-only) metadata generation has been added in #5552

The above adds some cli arguments to retrieve data and then use it and then allows generation for these.

What is missing is the following -

• RPCs are not generated based on the running chain (when overrides are applied)
• Runtime calls are not generated for the running chains (with overrides)

So basically, we need -

• where the argv endpoint is specified (as wss/ws), also retrieve version & RPCs, or
• where the endpoint is non-wss, also allow the specification of RPC/runtime JSON files, then
• supply these values to the RPC/runtime generation (as we do in the case for the Substrate/Polkadot/Kusama names)