docs: added multiversion_regex_builder

[dgarcia360]

Related issue https://github.com/scylladb/sphinx-scylladb-theme/issues/86

The new built-in function multiversion_regex_builder(versions) enables maintainers to define versions in an array instead of defining complex regex expressions.

Example

Before:

smv_tag_whitelist = r'\b(3.22.0-scylla|3.21.0-scylla|3.22.3-scylla|3.24.0-scylla)\b'

Now:

TAGS = ['3.21.0-scylla', '3.22.0-scylla', '3.22.3-scylla', '3.24.0-scylla']

It also adds a new command make multiversionpreview that launches a webserver to quickly preview the multiversion build.

Testing this PR

1. Run make multiversionpreview.
2. Open http://0.0.0.0:5500 in a new browser tab.
3. You should see folders for every version listed in TAGS (conf.py).