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/

Google will create an entityId with your domain, as described in #9.

Might not be complete, needs check with current G Suite interfaces.
But improves on the current situation.

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

* Remove 404 link to SSP news; this is covered on simplesamlphp.org now.
* Make screenshots link relative, so docs don't trigger insecure
  content warnings.
* Compress png images.

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

As per https://groups.google.com/forum/#!topic/simplesamlphp/nFXerlX839E

This is more than just cosmetics, as the rnd.feide.no links redirect
people to the SSP 1.5 documentation which is rather outdated.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3333 44740490-163a-0410-bde0-09ae8108e29a

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3208 44740490-163a-0410-bde0-09ae8108e29a

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2835 44740490-163a-0410-bde0-09ae8108e29a

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2711 44740490-163a-0410-bde0-09ae8108e29a

The documentation includes a spNameQualifier that matches the entityID
of the Google Apps SP, but we will use the entityID as the
SPNameQualifier by default.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2249 44740490-163a-0410-bde0-09ae8108e29a

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git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2206 44740490-163a-0410-bde0-09ae8108e29a

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1802 44740490-163a-0410-bde0-09ae8108e29a

Fixes issue 208

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1801 44740490-163a-0410-bde0-09ae8108e29a

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git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1591 44740490-163a-0410-bde0-09ae8108e29a

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1437 44740490-163a-0410-bde0-09ae8108e29a

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1299 44740490-163a-0410-bde0-09ae8108e29a

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1298 44740490-163a-0410-bde0-09ae8108e29a