Update folder structure to match sidebar

[GeckoEidechse]

Some of the source folder structure is a artifact of the old wiki structure. In particular all of docs/installing-northstar/using-northstar/ should be moved to docs/using-northstar/.

• README.md #173
• direct-connect.md #178
• launch-arguments.md #183
• server-browser.md #181
• commands.md #174
• gamemodes.md #177
• mods.md #175

[GeckoEidechse]

For anyone taking on this issue, I highly suggest making separate PRs for each page moved as there are a lot of links to update and limiting it to one page at a time reduces the change of missing any links and keeps diff easier/faster to PRs, ultimately speeding up the process ^^