docs: verify/bisect: proper headlines and more spacing

Various small improvements and fixes:

* Separate ref links from their target with a space for better
  readability.

* Add a proper heading for the note at the end of the step-by-step
  guide.

* Use proper 3rd and 4th level headlines in the reference section and
  add short intros for the 2nd level headlines that lacked one.

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/f59f0f235a2192ed93899a7338153e4cb71075f0.1712647788.git.linux@leemhuis.info