2446 update developer guideline docs

[JackEAllen]

Issue

Closes #2446

Description

Add to developer conventions documentation to describe Mantid Imagings version control processes to help new developers get up to speed.

Add a "Contributing" sub-header to the projects README.md to link to developer conventions and developer guide recourses.

Testing

Describe the tests that were used to verify your changes.

• Proof read
• Validated that Sphinx documentation builds correctly
  • build docs: make build-docs
  • run python server: python -m http.server
  • Open a browser and navigate the build docs to view them.

Acceptance Criteria

How should the reviewer test your changes?

• Content is grammatically correct.
• content clearly describes best practices for version control in Mantid Imaging, assuming no prior experience with version control or the Mantid Imaging project.
• Sphinx documentation builds correctly
  • Build docs: make build-docs
  • run python server: python -m http.server
  • Open a browser and navigate the built docs to view them:

  file:///<full>/<path>/<to>/mantidimaging/docs/build/html/developer_guide/explanations/developer_conventions.html

• README.md improves accessibility to contributing to the Mantid Imaging Project.

Documentation

N/A this PR does not require release notes.

[coveralls]

coverage: 73.622%. remained the same
when pulling 878932f on 2446-update_developer_guideline_docs
into 3f099d2 on main.