[DOCS] Add link to MIG metadata configuration spreadsheet to official docs about documentation

[ysuarez]

Lina and I created this issue, and we are still not sure where exactly in the page https://islandora.github.io/documentation/user-documentation/metadata/ we should add a link to the spreadsheet. Perhaps, we should create a new metadata related page to link for to the spreadsheet.

I can envision adding a section called the same thing as the name of the file: "Islandora Starter Site Metadata Configuration"

Then we could add one or two sentences explaining what the spreadsheet is. For example...

### Islandora Starter Site Metadata Configuration

This spreadsheet contains the details of the various fields, and taxonomy vocabularies, and relators configured for the starter site. 

This spreadsheet was created and maintained by the Islandora [Metadata Interest Group (MIG)](https://github.com/islandora-interest-groups/Islandora-Metadata-Interest-Group).

What do people think?

[rosiel]

addressed by #2295