Add document highlighting metadata fields.

ticket: https://github.com/Islandora/documentation/issues/2294

Purpose / why

The MIG created a lot of documentation about the default configuration of metadata fields in the Starter Site and are proposing it become part of the Documentation.

It is clear that this is merely the DEFAULTS and that you are encouraged to extend and change this however you like. However, seeing the defaults in a human-readable form has value.

This PR essentially adds 2 documents to the docs: first is this file (in markdown with HTML tables), the other is the spreadsheet linked at the top of the page.

The MIG is offering to monitor these documents as a standing agenda item and ensure any changes are updated in a reasonable time (up to 2 week delay). However we also invite anyone making changes to the starter site to create their own update PRs.

What changes were made?

New page! Lists all the fields in the Repository Item content type and their config.

Verification

Does the site build and look good? If you want to go further, this should match the starter site, the spreadsheet, and for the MODS xpaths, the MODS mapping.

Interested Parties

@Islandora/documentation
@Islandora/committers

Checklist

Pull-request Reviewer

Pull-request reviewer should ensure the following:

[ ] Does this PR link to related issues?
[ ] Does the proposed documentation align with the Islandora Documentation Style Guide?
[ ] Are the changes accurate, useful, free of typos, etc?

Person Merging

The person merging should ensure the following:

[ ] Does mkdocs still build successfully? (This is indicated by TravisCI passing. To test locally, and see warnings, see How To Build Documentation.)
[ ] If pages are renamed or removed, have all internal links to those pages been fixed?
[ ] If pages are added, have they been linked to or placed in the menu?
[ ] Did the PR receive at least one approval from a committer, and all issues raised have been addressed?

Islandora / documentation