docs abstraction & code formating

[2bndy5]

This is part of my crusade to

• abstract docs (main page and all "Related Pages" now live in the docs folder)
• fix formatting (mostly was just inconsistent indentation & missing docs interlinking between class defs and config defs)
• added a CONTRIBUTING.md file

NOTE: This lib is unlicensed.

Additional changes

• added a library.properties file (that limits to AVR arch only) for easy install with Arduino IDE.
• removed all printf.h files from examples' folders as the RF24 library has an updated compliant copy. fixes [request] remove printf.h files in example folders #35

[2bndy5]

CI rightfully failed because it tried to push the updated docs to upstream/gh-pages branch. Remember this is the only nRF24 lib that pushes doc updates for every commit (because releases aren't used). I expect this CI error to fix itself when the PR gets merged.

[2bndy5]

some hyperlinks might be broken until we merge nRF24/RF24#750 and release all the other RF24 libs

[2bndy5]

Nevermind. I don't see any broken links. the docs were deployed properly here.