Add docs styling and tweaks

[calvellido]

This PR adapts the docs templates to the proposed design.

Summary

• Decorated titles.
• Adapted docs and category cards according to the proposal.
• A refactor of the category pages. Now an index.md file is needed, but it can be mostly copypasted from oterh files juts modifying its title. The sidebar items are still autogenerated, like before.
• Code snippets using a proper colour theme and font family (JetBrains Mono).
• Adapted admonitions.
• Added Arrow styled icons.
• Arrow style themed sidebar
• Themed Mermaid diagrams.
• Some other small details and titbits, like the proper alignment of navbar, titles, toc, etc.

Preview

[serras]

Everything looks great!

@calvellido One small detail: would it be possible to make the six rectangles in Overview a bit shorter? I'm a bit worried people may not realize there's more content afterwards. Otherwise we could add some text that makes it clear that there's more information there.

[calvellido]

Everything looks great!

@calvellido One small detail: would it be possible to make the six rectangles in Overview a bit shorter? I'm a bit worried people may not realize there's more content afterwards. Otherwise we could add some text that makes it clear that there's more information there.

I made in 16b401d a small adaptation for them to have less padding. They now look like this: