improve UI/UX of documentation site

See https://www.figma.com/file/wyg7mMawGJhaXkiaNGH17H/Mithril-UX-Review?node-id=0%3A1

Some comments regarding the UX review

• The main website is generated with a tool docusaurus (therefore, we have limited control on how to display pages)
• Breadcrumbs usually link to the upper categories, up to the home page of the website (which is the behavior implemented in docusaurus)
• The Rust documentations are also automatically generated and use the same theme as all other libraries of the language (with which developers are used to work)
• The content is not final yet and under redaction. Also some diagrams and informative blocks will be added

Improvements done

• Improve footer by adding missing links (including a link to contact us with Github Discussions)
• Improve footer on mobile version by centering its links
• Add links on blocks in the homepage
• Fix capitalization in Developer Docs
• Use https urls for rust documentations
• Add missing rust documentations in the menu
• Fix confusing menus in sidebar by reorganizing contents
• Enhance colors with informative blocks with Admonitions
• Enhance colors with an announcement bar
• Enhance colors with a different primary color for the theme

Improvements to do later

• Translation in other languages (but first we need to complete English version)
• Video tutorials and guides
• Align to the right the toggled navbar on mobile version (if possible)