grml-docs/startpage.html + templates/boot/isolinux/f10: rework and update instructions

[mika]

Re grml-docs/startpage.html:

• 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

And in templates/boot/isolinux/f10 I stumbled upon references to freenode

[zeha]

Nice