Improve the UX of the docs website

[Gedochao]

• [x] fix the search feature
• [x] add categories level for guides & cookbooks
• [x] shorten guides & cookbooks titles in the links tree
• [ ] generate sitemap & analyse it
• [ ] look for other improvements

[MaciejG604]

Some sections from Reference could probably be merged with Guides e.g.:

• [ ] Reference > Proxy authentication and Guides > Repositories and HTTP Proxies
• [ ] Reference > Dependency format and Guides > Managing dependencies / Updating dependencies

Also Under the hood and Guides > Internals have some similiar elements