Prepare docs for 0.24.0

Summary: This updates the documents to prepare for the minor version bump, but does not change the version yet to not confuse the users. Only the CMakeLists.txt version remains to be updated when the release is ready.

Test Plan: Read the docs, compare with D9854.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9873