[Docs] Clean up the contributing README

[hmellor]

• Add the ability to skip API ref when building the docs locally to make it faster with API_AUTONAV_EXCLUDE=vllm (useful if you don't need the API ref for what you're doing)
• Fix a sentence in /docs/getting_started/installation/python_env_setup.inc.md
• Restructure the contributing README into the following structure to better match the order a new contributor is likely to need to know the information:

...

## Developing

Clone repo and editable install

### Linting

How to set up pre-commit

### Documentation

How to build the docs

### Testing

How to run the tests

...

[gemini-code-assist]

Code Review

This pull request significantly improves the contributor documentation by restructuring the contributing/README.md for better clarity and logical flow. It also introduces a useful feature to speed up local documentation builds by allowing the exclusion of API reference generation. The changes are well-thought-out and enhance the developer experience. I've found one area in the testing instructions that could be clarified to prevent confusion for new contributors.

[punitvara]

Yes contributing README looks much better and helpful now. If you want you can specifically mention steps before raising new PR. From setting up virtual env, run pre-commit to running any other pytest etc.