RPC: Generate documentation from CI and put it up somewhere sensible

When the dust settles we'll have the following new generated documentation

• The rpc-book mdbook.
  • To generate this, install mdbook and mdbook-linkcheck.
  • In doc/dev/rpc-book, run mdbook build. The output is in book.
• Documentation for the C API.
  • For this we can start by just using the header file, but in the long run we should use some kind of C documentation formatter (doxygen? Other) to put it into HTML.
• The RPC method reference
  • The build process here is kinda janky. Basically, you have to follow all the build and envvar instructions in rpc-book/src/getting-started.md, and then run ./maint/rpc-docs-tool FILENAME_OUT.html. (You can also specify an md file.) Maybe I should automate this a bit more.
• The Python library API
  • Not quite sure what to do here; we should autogenerate something probably, but I don't know what people like to use in pythonland.

We should have a CI task that builds all these nicely and puts them somewhere sensible.

After that, we can add links from each of these sources to the others as relevant.