Add /current/ to RPC docs

[Sjors]

Google tends to serve old versions of our RPC docs: https://www.google.com/search?q=walletprocesspsbt&oq=walletprocesspsbt

We can probably improve this by having a /current/ URL for each method, as well as the RPC docs landing page. We can still have links to old versions for those who need it.

Alternatively we could have the RPC doc landing page redirect to the latest version. That seems to be how readthedocs.org handles it, e.g. https://wally.readthedocs.io/