Fix top-level index links to RPC docs

Summary: Actually make the link point to the right sub-index

Test Plan:
make serve
Navigate to http://localhost:4000/doc/ and click the link for 0.21.12
It should go directly to the expected index page for that version.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7178