docs:opkg: clean up RST

* Fixup minor RST layout issues.
* Use inline literal markup for inline code and applications, rather
  than bold.
* Refer to IPKs and DEBs, instead of .ipks and .debs in most places.
* Reflow prose to line-break on sentence ends, for easier diffing.
* Minor content changes for clarity.

Signed-off-by: Alex Stewart <alex.stewart@emerson.com>

Author: Alex Stewart