Documentation check + update

[jacobcook1995]

This PR is consists of a very large number of documentation fixes. Some of these for minor bugs/problems, but most of these are making the docs less specific to the SAFE project.

As part of this I made a safedata at SAFE page. At the moment it does feel like a slightly random collection of information, so let me know if you have any suggestions for improvements there.

Also @davidorme, I add two TODOs in docs/using_safedata/overview.md to create/extend the use case recipes for uploading non-Excel files, and to update an existing data set. I don't know if they should be added onto this PR, but think they should be done before the release of 3.0.0

Fixes #77 Fixes #48

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (657f9ea) 68.74% compared to head (8b66406) 68.74%.

Files	Patch %	Lines
safedata_validator/entry_points.py	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release/3.0.0 #79 +/- ## ============================================== Coverage 68.74% 68.74% ============================================== Files 12 12 Lines 3711 3711 ============================================== Hits 2551 2551 Misses 1160 1160 ```

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