Command formatting (see #10213)

This PR addresses a discussion thread initialized in #239 and stored in ticket 10213. 

Unification of command markup is done as follows:

- after discussion with @hflynn, all non OMERO command now use the literal `` :literal:`command` `` role (double back quotes) instead of the `:command:` role. All the occurrences of the latter role are reviewed and replaced by the adequate form in the code.
- to unify the command formatting, the node of the `:omerocmd:` role is changed into `nodes.literal`

Additionally, to solve the OS dependency of CLI commands, this PR implements the following:
- when referring to a command, `` :omerocmd:`admin restart` `` now displays `omero admin restart` in the text and generates a `omero admin; restart` entry in the index table. The latter change allows the proper grouping of all the CLI command in the index.
- if ones needs to specify the file separator, the `:file:` role is used assuming the sentence refers to a (relative) file path, e.g. ``:file:`bin/omero` `` or ``:file:`bin\\omero` ``

/cc @bpindelski, @ximenesuk