Change documentation from recommending 2048 bit keys to using 3072 bit
keys.

Given that we're recommending people generate ten-year keys, 2048 bit
keys are probably a bit short. Almost all commercial certificate
authorities now recommend 4096 bit keys, and eduGAIN requires 3072 bit
keys for new federations.

This change aligns the SimpleSAMLphp documentation with the lower
eduGAIN requirement, since that still meets most standards bodies
recommendations for 2028 (ten years from now). cf
https://www.keylength.com/

Change the extension of the documentation files from .txt to .md so that they can processed as markdown and displayed in github.

(being respectful with occurences that might change the behaviour, i.e. default database prefixes)

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2713 44740490-163a-0410-bde0-09ae8108e29a

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2617 44740490-163a-0410-bde0-09ae8108e29a