[fpc-pascal] Docs style update

[Michael Van Canneyt]

Hi,

In an effort to improve the readability of the online docs, 
I've instructed Claude to create various CSS styles for the documentation.

You can view the 3 which I retained here:

https://www.freepascal.org/~michael/docs-demo/docs/

I'd like to sollicit some feedback on which is considered the most readable of the
three: book, compact or modern.


Note this concerns only the latex-style manuals, not the reference
documentation - that is generated by another mechanism 
(which will also be updated)

Note that only CSS changes are involved, so no 'structural' changes.

Michael.