[Documentation] add documentation functions to makefile

[veewee]

This PR adds functions to the makefile for generating the docs:

docs-generate                  regenerate docs
docs-check                     checks if docs are up to date

The docs-check now gets executed when running make check locally.
This will avoid github actions failure in advance until #206 is in place.

[coveralls]

Pull Request Test Coverage Report for Build 1316226145

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.936%

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Totals
Change from base Build 1311016514:	0.0%
Covered Lines:	3119
Relevant Lines:	3121

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💛 - Coveralls