feat: add fetch_multiversion_configuration

[dgarcia360]

Related issue: scylladb/scylladb-docs-homepage#30

Adds a helper to get multiversion data from a remote URL.

To test the changes, we'll build multiple versions using data from the following URL:
https://gist.github.com/dgarcia360/d18edbfac5e2fc1443ba0819840e2fd1

{
    "tags": [],
    "branches": ["master", "branch-1.8"],
    "latest": "branch-1.8",
    "unstable": ["master"],
    "deprecated": []
}

How to test

1. Checkout master and branch-1.8 branches:

git checkout branch-1.8
git checkout master

2. Checkout this PR:

 gh pr checkout 1303

3. Edit docs/conf.pyglobal variables section as follows:

VERSIONS_URL = "https://gist.githubusercontent.com/dgarcia360/d18edbfac5e2fc1443ba0819840e2fd1/raw/dcf460800849f5bc623982532edeb70499f0e3a9/gistfile1.json"
MULTIVERSION_CONFIG = fetch_multiversion_configuration(VERSIONS_URL)
# Builds documentation for the following tags and branches.
TAGS = MULTIVERSION_CONFIG.get("tags", [])
BRANCHES = MULTIVERSION_CONFIG.get("branches", [])
# Sets the latest version.
LATEST_VERSION = MULTIVERSION_CONFIG.get("latest", "master")
# Set which versions are not released yet.
UNSTABLE_VERSIONS = MULTIVERSION_CONFIG.get("unstable", [])
# Set which versions are deprecated
DEPRECATED_VERSIONS = MULTIVERSION_CONFIG.get("deprecated", [])
# Sets custom build.
FLAGS = ["theme"]

This change will read configuration versions from an external file.

4. In the same conf.py, set smv_remote_whitelist to None:

smv_remote_whitelist = None

This ensures the multiversion build considers only the local branches cloned in Step 1.

5. In the terminal, run the multiversion preview:

   make multiversionpreview
   

6. Verify docs build with multiversion enabled. The branch-1.8 version should be listed as available in the documentation.

Next steps

Release a new version of the theme and configure open-source and enterprise repo to read from remote files for multiversion config.

[annastuchlik]

Run with
from sphinx_scylladb_theme.utils import multiversion_regex_builder, fetch_multiversion_configuration

[annastuchlik]

LGTM