DOC: Installation instructions in the developer start guide outdated

[Dhruvanshu-Joshi]

Issue with current documentation:

The current documentation for the dev_start_guide directs contributors to run the following commands:

pytensor/doc/dev_start_guide.rst

Lines 176 to 179 in 30b760f

	.. code-block:: bash
	pip install -r requirements.txt

pytensor/doc/dev_start_guide.rst

Lines 208 to 211 in 30b760f

	.. code-block:: bash
	pip install -r requirements-rtd.txt

However, we have long dropped the use of requirements.txt and requirements-rtd.txt and hence this needs to be rectified.

[ricardoV94]

Yup, instructions should you conda + environment.yml I think