Improve the `Makefile` commands relating to the docs

[drbenvincent]

A few things are clunky:

• We have a separate ./docs/Makefile which gives you make html and make clean commands. It's not a massive deal but it means you have to cd into docs first and I often forget to do that, or to move back into the project root afterwards. We should remove ./docs/Makefile and get the Makefile in the project root to do all the work.
• That ./docs/Makefile is pretty cryptic. I think I originally got it from some external documentation, so I don't really understand how it works. We can maybe use the pymc-marketing Makefile as inspiration. But it has clear commands html and cleandocs, which is nice.
• The current way we clean the docs cd docs; make clean removes everything from ./docs/build, which is fine. However I am finding in practice you also need to manually remove the contents of ./docs/source/api/generated/, otherwise you can run into errors when building the docs locally.
• We'd also want to update the few relevant parts in CONTRIBUTING.md

[drbenvincent]

Possibly of interest to @Sohaibameer204 ?

[Sohaibameer204]

@drbenvincent Sure! You can assign this to me.

[Sohaibameer204]

@drbenvincent i updated the root makefile and deleted which is inside the doc folder and updated the contributing.md file as well , now need your approval . Did i do as what you wanted?

[drbenvincent]

Thanks very much. I have a family weekend so I might not be able to check until Monday. But looks good from what I can see 👍🏻

[drbenvincent]

Closed by #483