grml-docs/startpage.html: rework and update instructions

* We don't ship xlinks2, so drop it from the browser list
* Iceweasel no longer exists
* There's no point in providing usage instructions for the command line
  browsers, other than how to exit them (if someone really ends up
  there without being used to w3/links)
* Replace all references to http with https
* Let's drop tldp.org / Linux Documentation Project (unsure whether
  this is relevant for anyone nowadays?), and instead link to Grml Wiki
* We moved our IRC from freenode to oftc, update accordingly
* While at it, misc rewordings and section adjustments, to reduce text
  on page