doc: drop single quote references

Many documents relied on single quotes to create references, e.g.
`my_reference`. This is possible because `default_role = "any"` is
enabled in Sphinx conf.py. However, this method comes with its problems:

- It mixes all domains together, so it's not clear to what are you
  referencing: a document? a Kconfig option? a C function?...
- It creates inconsistencies: in some places explicit roles are used
  (e.g. :ref:`my_page`) while in some others not.
- _Conflictis_ with markdown. Single quotes are used for literals in
  Markdown, so people tend to use the same syntax in Sphinx, even though
  it has a different purpose.

Usages have been found using `git grep ' `[^`]*` ' -- **/*.rst`.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
11 files changed