Docs: Remove serverless

[Lorak-mmk]

For now Scylla Cloud Serverless does not exist, so there is no point in keeping its docs. If it is ever brought back, we can restore the docs from git.

Fixes: #487

Note: I was not able to test the change locally because of #488

Fixes

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.