Fix docstring references in the manual

[mortenpi]

Try to fix #2232.

[mortenpi]

Ok, this should now actually resolve all the docstring referencing issues:

• Remove references to docstring that don't exist from the manual
• Remove at-refs to functions that don't have (DataFrames-specific) docstrings anymore
• Add in all the docstrings that exists, but were not included in the manual.

I also enabled strict = true for makedocs -- this should fail the documentation build if a PR introduces some new referencing problems.

[bkamins]

Thank you - I will merge this once the CI passes and later I will rewrite the whole page to be more structured.

[bkamins]

Thank you!