minor doc improvements/additions

[alessandrofelder]

Description

What is this PR

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

Why is this PR needed?

• we've made some of the atlas_generation code depend on optional installation, so we need to update the documentation accordingly
• the atlas_generation documentation does not reference improvements to validation (e.g. https://github.com/brainglobe/brainglobe-atlasapi/pull/249) and visual inspection made recently.

What does this PR do?

• specify that the atlasgen optional dependency is needed to contribute an atals
• provides a sample script for visual inspection of atlas
• minor clarifications in text
• more in-depth validation documentation to be contributed in a separate PR.

References

Closes #203

How has this PR been tested?

CI, and proofreading by PR author.

Is this a breaking change?

No

Does this PR require an update to the documentation?

This is one.

Checklist:

• [ \ ] The code has been tested locally
• [ \ ] 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