docs: Update contribution guide for main branch workflow

[john-sandall]

Context

This PR assumes the development workflow no longer uses the development branch as it does not exist. It is noted that https://github.com/EpistasisLab/tpot2/tree/dev does exist but the last commit is 7 months old i.e. older than https://github.com/EpistasisLab/tpot2/tree/main ... if this assumption is incorrect and the development branch will be reinstated, then this PR can be updated accordingly. The section around pip install -e . may still apply.

What does this PR do?

This PR updates the contribution guidelines (docs/contribute.md) to reflect the assumed development workflow which seems to use the main branch for both stable releases and active development. It removes references to the deprecated dev and development branches and clarifies the developer setup process, directing users to the developer installation instructions.

Where should the reviewer start?

Review the changes in docs/contribute.md.

How should this PR be tested?

Read through the updated docs/contribute.md to ensure the instructions are clear, accurate, and reflect the current repository practices (targeting main, using pip install -e . for setup).

Any background context you want to provide?

The previous guidelines referred to dev/development branches which are no longer active, and contained conflicting information regarding the developer environment setup. This led to confusion for potential contributors.

What are the relevant issues?

Closes #1372

Questions:

• Do the docs need to be updated? Yes, this PR updates them.
• Does this PR add new (Python) dependencies? No.