Move files and fix links

[zuiderkwast]

Until a few years ago, the documentation was mainly written by Salvatore and had a simple structure with URLs on the form "/topics/pagename" for all pages except commands. This is how it used to look like until some time in the beginning of 2022: https://web.archive.org/web/20211216094009/https://redis.io/documentation

We've decided (in a core team meeting) to restore this structure. It gives back a feeling from the good times. There are still many links to these URLs in places like comments inside valkey.conf, the module API docs in module.c and other places, so it's a bonus that these links will work again.

I'm working on a script to rename the files and to fix the links within the repo. I'm opening this issue and assigning myself mainly to show what I'm working on. Some minor questions though:

• I want to change to relative links and making the links point to .md files (i.e. make the links like ../commands/hget.md instead of /commands/hget). This would make it possible to click on the links in the GitHub pages in the valkey-doc repo. Is this OK?
• What to do with links like /commands/?group=sorted-set, i.e. links to commands categories? Shall we generate static pages for each of these groups or should we instead generate links like /commands/#sorted-set and do the filtering on the browser side? The command index page could even be a static page with anchored headings for each command group. WDYT?

@stockholmux