Review developer/contributing documentation for completeness

[mmcky]

Context

The quantecon.org website is removing its developer instruction pages (see QuantEcon/website#201), since developer docs were migrated to this repo in #740. The website's Contribute buttons now point at the rendered contributing guide:

https://quanteconpy.readthedocs.io/en/latest/contributing.html

This is a request to review that the developer docs here fully cover what the website pages used to provide.

Removed website pages and their coverage

Removed page	Topics	Covered by docs/source/contributing.rst?
/python-developers/	Conda dev env, tests, documentation, package structure, the QuantEcon.py API	Partially — the Package structure and API sections have no equivalent
/wiki-py-conda-dev-env/	Step-by-step Conda development environment setup	Summarised only
/wiki-py-docstrings/	Detailed numpydoc docstring guide (~280 lines)	Unclear — worth checking nothing of value was lost
/wiki-py-unit-testing/	Unit testing guide — test naming, assertion methods, examples (~130 lines)	Unclear — worth checking nothing of value was lost

The full text of the removed pages is retrievable from the website repo history if anything needs porting.

Suggestion: add a root CONTRIBUTING.md

The repo currently has no root CONTRIBUTING.md (or .github/CONTRIBUTING.md), so GitHub shows no contributing-guidelines link on new issues/PRs. A short stub that links to the rendered readthedocs guide would be enough.

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