.gitignore new versioned doc files

Summary:
Calling scripts/fetch_documentation.sh leaves unstaged files in the repo. We are no longer tracking
new versioned docs under source control, so we should ignore them.

Note: The versions currently under source control are not supported by the fetch script, so they need to remain
until 0.23.0 is released, and then they will be obsolete and can be safely removed.

Test Plan:

./scripts/fetch_documentation.sh

Pre-patch, git status shows unstaged files under _doc. Post-patch it does not.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7460