feat: add mermaid.js support

[dgarcia360]

Adds Mermaid.js support.

How to test

1. Clone the PR and preview the docs.

2. Open http://127.0.0.1:5500/examples/diagrams/

3. Verify the sample diagram renders as expected:

   

[dgarcia360]

@annastuchlik ready for review

[annastuchlik]

@dgarcia360 I'm getting some errors:

Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/home/mat77/.cache/pypoetry/virtualenvs/sphinx-scylladb-theme-LGaAq45G-py3.10/lib/python3.10/site-packages/sphinx/config.py", line 529, in eval_config_file
    exec(code, namespace)  # NoQA: S102
  File "/mnt/c/Users/AS/Documents/git-repos/temp/sphinx-scylladb-theme/docs/source/conf.py", line 14, in <module>
    MULTIVERSION_CONFIG = fetch_multiversion_configuration(VERSIONS_URL)
NameError: name 'fetch_multiversion_configuration' is not defined

[dgarcia360]

NameError: name 'fetch_multiversion_configuration' is not defined

It looks like you were trying to build this PR with the conf.py edits we applied for #1303 during our latest call.

Could you please try again, making sure there are no edits in conf.py?

[annastuchlik]

Sorry about that. I've checked this PR on a new workspace and now it's working. Thanks.

[annastuchlik]

LGTM