Help: Restructure the find_package() docs

The explanation of the basic and full signatures was interwoven tightly with that of the Module and Config search modes. Config mode is supported by both signatures, which made the structure of the command documentation a bit confusing.

Add a section at the start which unambiguously describes the two modes, including the fallback functionality between them. Move the text about this out of the basic signature section, leaving just a description of the basic syntax with a note that it is supported by both search modes. This makes the basic signature section more focused.

Drop the Module and Config mode parts of the titles for the basic and full signature sections. Those are now more precisely covered by the new opening section, so the Basic Signature and Full Signature sections are now clearer in what they cover.

Swap the order of the Search Procedure and Version Selection sections to improve the logical flow. Also add "Config Mode" to their titles to make it clearer what they apply to. Add a note at the start of both sections to highlight their relationship to the basic and full signatures.