Expand the developer documentation

Description

What is this PR

[ ] Bug fix
[x] Addition of a new feature
[ ] Other

Why is this PR needed?

As a novice (and even as an experienced contributors), it's currently not easy to get a quick, good overview of the how the BrainGlobe code base is organised.

What does this PR do?

This PR represent a major fleshing out of the developer docs.

This PR adds a high-level introduction to the code base for new and existing contributors. It includes:

guiding principles (with explanatory examples)
naming and organisation of packages in org
suggested default software architecture for brainglobe tools

This PR further

incorporates the high-level intro into the wider website
adds some knowledge that was living in our heads to website text, this includes:
- handling of user data (GIN, pooch, .brainglobe folder)
- documentation around test data
- axis ordering docs
- a link to the core development project board
- release process across repos (particularly interested in @willGraham01 looking at this part!)

Note that some of the information about the tool names is slightly ahead of our renaming/merging tool repos schedule 😬 It considers https://github.com/brainglobe/BrainGlobe/issues/43 as already addressed.

References

Closes https://github.com/brainglobe/BrainGlobe/issues/47 Closes https://github.com/brainglobe/brainglobe.github.io/issues/8 Closes #81

https://github.com/brainglobe/BrainGlobe/issues/16 to be tackled separately to this.

How has this PR been tested?

Inspection of local build of website.

Is this a breaking change?

Does this PR require an update to the documentation?

This PR is an update to the documentation

Checklist:

[x] The code has been tested locally
[n/a] Tests have been added to cover all new functionality (unit & integration)
[x] The documentation has been updated to reflect any changes
[x] The code has been formatted with pre-commit

brainglobe / brainglobe.github.io