Major update to the RonDB documentation

My colleague Vincent has spent some time improving the RonDB documentation. 

New/rewritten chapters/sections are:

• Main page: https://docs.rondb.com
• Installing: https://docs.rondb.com/rondb_installation/
• Local Quickstart: https://docs.rondb.com/rondb_quickstart_local/
• Start Distributed: https://docs.rondb.com/rondb_programs/
• Recovery (entire chapter): https://docs.rondb.com/rondb_high_availability/
• Two-Phase Commit Protocol: https://docs.rondb.com/rondb_nonblocking_2pc/
• Transaction Model (only ACID section, further PR incoming) https://docs.rondb.com/intro_transactions/

Further UI changes/fixes:

• Added dark mode
• HTTP links are visible again
• Recognition of programming language in code snippets (using Lua filter)
• Order & naming of chapters
• A number of new images based on our Cheetah logo