Add admin links, vale fixes, and add docs on using registration forms

This PR:

Fixes a bunch of Vale issues on the pages I edited (as this content was written in the UI without using Vale 😊 )
Adds the admin links to various sections which mention admin logins (per feedback from @domidc )
Fixes the issue we found in https://github.com/mautic/Community-Portal/issues/31#issuecomment-1907923785 which was a lack of understanding on creating forms - added some docs on this to the meetings page.

mautic / mautic-community-handbook