Add Developer Onboarding Plan and Supporting Documentation

[ashleymeigh2]

Issue

Closes #2408

Description

This PR adds the onboarding plan and supporting documentation to assist new developers in setting up their environment and getting familiar with the project. Key elements of the onboarding plan include:

Goals for the onboarding process.
Week-by-week overview of tasks and milestones.
Core skills development with detailed links to resources for Python, PyQt, and Neutron Imaging.
Detailed guidance on Mantid Imaging's MVP architecture.
Supporting links to external documentation and case studies.

Testing

Verified the formatting of the onboarding document in RST format.
Checked that all external links included in the documentation are functional and relevant.
Previewed the rendered documentation to ensure clarity and readability.

Acceptance Criteria

Review the added onboarding document for accuracy and completeness.

Documentation

The onboarding plan has been added to the documentation

[samtygier-stfc]

I don't think the onboarding plan needs to be in the docs, as it varies by the needs/experience of the developer.
But the list of resources is useful, so lets keep that.