Docs: Updates with an example on how to check for black & white pages.

[jamie-lemon]

Also some updates to file conversion section & docs README

[JorjMcKie]

The wording ...

A simple check to determine if a page is black and white or has color can be done by checking for whether a page is monochrome.

... is logically incorrect, because the negation of "black & white" is not the same as "page has colors".
A page with many "shades of gray/grey" is also not black & white but neither shows colors.

Black & white means that everything on the page is either pure white or pure black. This is what we are actually checking. If the result is True then we can conclude that the page has definitely no colors. But in case of False we cannot conclude that there are colors.

A page without colors is one where all pixels have R=G=B when the page was rendered as RGB.

Suggestion:

A simple check to determine if a page is pure black and white ("monochrome"). Non‑monochrome means the page has multiple gray levels or colors.