docs: add sample site

[dgarcia360]

Motivation

Adds a sample docs site to the project.

How to test

1. Make sure you meet the prerequisites.
2. Clone the PR.
3. Go to the docs folder: cd docs
4. Build the docs: make preview
5. The site should render at http://127.0.0.1:5500/

Next steps after merging

1. Add content to the site (maintainers)
2. Request zendesk tag for this project, optional (@annastuchlik)
3. Configure the subdomain (@dgarcia360)

4. Configure GitHub Pages (maintainers)

   Could you please enable the following option?

   In the repository’s Settings, navigate to the Pages section and select GitHub Actions as the deployment source.

   Thanks!

[guy9]

@annastuchlik please review

[guy9]

Even though it's empty, check that it works and we will add content after the merge

[annastuchlik]

LGTM @tarzanek @julienrf Please merge this PR and configure GitHub pages (as shown in the description of this PR).

Background: We want to create and publish documentation for Scylla Migration in the same way as we do for all other Scylla projects. This PR adds the required components, such as the necessary folder structure (docs, source, utils), a Makefile to build the docs with Sphinx, the workflows to build the docs and PRs, a list of dependencies, etc.

Once it's done, we will be able to add content to the docs folder and publish it on https://migrator.docs.scylladb.com.

[tarzanek]

@dgarcia360 what custom subdomain will be needed in Pages?

[tarzanek]

ah, okay, so I read @annastuchlik comment and subdomain will be migrator.docs.scylladb.com I configured it and triggered a build

[tarzanek]

there was one more option that picked workflow, but I assume it will be picked up after merge automagically