More ToC QoL features/fixes

[benlubas]

closes #1475

features:

• configurable max width
• automatically open/enter ToC on neorg buf enter
• close ToC when norg buffer is closed
• exit nvim when ToC is the last window (similar to how help windows function)
• set winfixbuf on the ToC window

These are all configurable behaviors.

fixes:

• when a ToC is open and it's norg buffer has been removed, it will no longer throw an error when you try to jump to a heading, instead it opens a new buffer, re-sets the ToC width, and placed your cursor in the right spot.
• opening a ToC from within a ToC window will no longer break the window

---

There's also a random completion documentation fix in here b/c I'm lazy and don't want to open a separate PR.

[benlubas]

Found a bug relating to closing/opening windows 'at the same time' I'll take a look tomorrow