Change the theme look and feel to match the ScyllaDB website

[annastuchlik]

Modify the theme to better match the webstie: https://www.scylladb.com/

This is a task for a ScyllaDB designer, so it belongs to the Design_tasks project.

Refs: https://github.com/scylladb/scylladb-docs-homepage/issues/11

@timkoopmans FYI

[dgarcia360]

Some other tasks that the designer might want to take into account:

• https://github.com/scylladb/sphinx-scylladb-theme/issues/621
• https://github.com/scylladb/sphinx-scylladb-theme/issues/257
• https://github.com/scylladb/sphinx-scylladb-theme/issues/251
• https://github.com/scylladb/sphinx-scylladb-theme/issues/358
• https://github.com/scylladb/sphinx-scylladb-theme/issues/938
• https://github.com/scylladb/sphinx-scylladb-theme/issues/859
• https://github.com/scylladb/sphinx-scylladb-theme/issues/934
• https://github.com/scylladb/sphinx-scylladb-theme/issues/945
• https://github.com/scylladb/scylladb/issues/18905