doc/system: Add documentation for NXInit

[acassis]

Summary

Add documentation for the Init component, including syntax explanations and simple examples.

Impact

User will be a documentation for reference

Testing

$ make html
...
copying images... [100%] _static/images/menuconfig-debug.png
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded.

The HTML pages are in _build/html.

[cederom]

My remarks:

• create common description of init describing possibilities.
• create dedicated page for nsh/init.d and nxinit with detailed description and crosslink to the top document.
• there is a .. warning:: rst mark that we can use to mark nxinit as work-in-progress experimental solution.

[acassis]

My remarks:

* create common description of init describing possibilities.

* create dedicated page for nsh/init.d and nxinit with detailed description and crosslink to the top document.

* there is a `.. warning::` rst mark that we can use to mark nxinit as work-in-progress experimental solution.

@cederom I liked the idea, but this PR is focused on NXInit. Could you please create an Issue with this suggestion?

[cederom]

Thank you @acassis :-)