Incomplete "Creating custom schemas" Tutorial

[JFRudzinski]

[ahm531]

I have removed this page from the navigation (the file still exists). The reason tha that two new pages were written by Siamak, which are currently under review in this PR #7
The two pages cover tuorials on creating custom ELN shema using YAML, and careting a schema for a tabular parser.

[JFRudzinski]

Do we close the issue then? Or add closed by in the corresponding PR

[ahm531]

I have done the later.