Added documentation for Active-Active

[vladvildanov]

Pull Request check-list

Please make sure to review and check all of these items:

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

Description of change

Please provide a description of the change here.

[elena-kolevska]

Some suggestions for a better organisation of the initial part

[petyaslavova]

The spellcheck action is still failing. Some of the flagged words are standard abbreviations and should be added to the pyspelling.filters.html ignores filter in redis-py/.github/spellcheck-settings.yml.