Assorted changes for the recently added document.

Improvements:

* Add instructions for installing required software on Arch Linux.

Fixes:

* Move a 'git remote add -t master stable [...]' from a totally wrong
  to the right place.

* Fix two anchors.

* Add two required packages to the openSUSE install instructions.

Fine tuning:

* Improve the reference section about downloading Linux mainline sources
  to make it more obvious that those are alternatives.

* Include the full instructions for git bundles to ensure the remote
  gets the right name; that way the text also works stand alone.

* Install ncurses and qt headers for use of menuconfig and xconfig by
  default, but tell users that they are free to omit them.

* Mention ahead of time which version number are meant as example in
  commands used during the step-by-step guide.

* Mention that 'kernel-install remove' might do a incomplete job.
Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <6592c9ef4244faa484b4113f088dbc1beca61015.1709716794.git.linux@leemhuis.info>