Updates to contributing.md and readme

[keegansmith21]

Changes to some existing docs. These need updating as they're out of date with the current state of the repo. I also make reference the the platform's dependent repositories as they use the same systems.

Notes for post-merge:

• Rename develop branch to main
• Update the two dependent repositories with a contributing guide that references this one

[keegansmith21]

@jdddog Please have a look over the changes and current state of the changed docs. It would be good to get your input - please make any changes you think necessary

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (379ed90) 87.05% compared to head (843dfd0) 87.05%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #640 +/- ## ======================================= Coverage 87.05% 87.05% ======================================= Files 34 34 Lines 4713 4713 Branches 574 574 ======================================= Hits 4103 4103 Misses 483 483 Partials 127 127 ```

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