GPXIN-35: Bring Documentation up to date

There's quite a bit missing from the documentation, so initially need to have a crawl over and identify what needs to be added. Some documentation work was originally done in GPXIN-21 and GPXIN-8

So far I can see that the following needs doing

- Add section on installing with composer
- Add section on suppressing data (not mentioned at all)
- Add section on grabbing a copy of the metadata
- Add section on Route stats (GPXIN-33)

There's probably quite a lot more missing too. Bits and pieces have been added over time, but it's probably worth looking at fully refactoring the docs.