Add CMake Find module for LibDeflate (#1483)
* Add CMake Find module for LibDeflate
Replaced `FindDeflate.cmake` with a new, rewritten, `FindLibDeflate.cmake`.
Its name matches with the config module that ships with newer `libdeflate` packages; this makes a smooth transition easier.
The new Find module also checks for the correct version number.
* Reformat Find module
Reformatted `FindLibDeflate.cmake` using `cmake-format`, ignoring the comment header.
* Convert comment header to RST
Modern comment headers use ReStructuredText as format. Rewrote the header, adhering to this modern standard.
* Bug fix
Pass correct variable to find libdeflate in non-standard directory.
Documentation correction.
* Some tweaks
Made version variable non-cached.
Reformatted documentation.
