doc: extensions: add custom role 'module_file'

Added another custom role `module_file` which functions like
`zephyr_file` but for any module in the west manifest.

The new role must take a module as an argument. Example syntax:

```rst
:module_file:`module-name: path/within/module`
```

or with custom link text:

```rst
:module_file:`my link text <module-name: path/within/module>`
```

This change also moves the hardcoded project name and link to
github to `doc/conf.py` by adding the configuration values
`link_roles_manifest_project` and `link_roles_manifest_baseurl`

Signed-off-by: Gaute Svanes Lunde <gaute.lunde@nordicsemi.no>