[WEBSITE] Use the updated rpc-doc target to build the RPC documentation

Summary:
The RPC docs are generated by the fetch-documentation.sh script. The
ABC commit rABC15caaf8d1f27435b17c8a0b8ff0b2bdf58ad5049 changed the
behavior of the doc-rpc by making it self contained, allowing for
greatly simplifying the fetch script. This also makes it incompatible
with < 0.22.2 versions, so the previous method need to be retained until
these version are deprectated. It is also improved using the feedback
from the abc commit.

Test Plan:

make doc

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7415