Add customizable layout options per page

[dgarcia360]

Closes #229 (comment)

How to test this PR

1. Clone this PR. For more information, see Cloning pull requests locally.

2. Enter the docs folder, and run:

make preview

3. Open http://127.0.0.1:5500/ with your favorite browser. The home page layout must look the same as https://sphinx-theme.scylladb.com/stable/. Furthermore, the internal page layout (e.g. Installation) must look the same as before https://sphinx-theme.scylladb.com/stable/getting-started/installation.html.

Releasing this change

This PR introduces breaking changes. To ensure it doesn't affect other projects with the new theme, I'll release a new version increasing the minor number. Unfortunately, this will prevent projects from receiving the update automatically.

After releasing 1.1, I'll submit PRs for the projects care-pet and sphinx-scylla-monitoring to install the latest version.