docs: ARC: Improve readability

docs: ARC: Improve readability

There were some things which made rendered document
look not very elegant. That was because:

1. Numbered lists were formatted in more of Markdown way
   rather than true reStructuredText and so were displayed
   as a plain text with leading numbers.

   Well, moreover numbered lists were not needed as in all cases
   we were just listing a couple of options w/o any intention to
   follow any particular order, so a simpler unordered list fits
   better and looks cleaner.

2. URL's of external resources were added as they are
   (which is OK in a plain text, but make not much sense in
   a HTML where we may use more human-friendly link names
   with URL's hidden.

3. Some URL's had trailing slashes which were not really needed

Fix all items from above!

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Vineet Gupta <vgupta@kernel.org>
Link: https://lore.kernel.org/r/20211202215747.19923-1-abrodkin@synopsys.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>