Docs/42/add relevant collections for needs assessment

[ll-zerr]

This PR adds a section to collections.md that lists the collections related to the needs assessment.

It contains the following content:

• a list of collections and the associated links to documentation regarding the collection
• a list of collections that exist as relation fields to the NeedsAssessment.Need collection
• a subsection addressing where to view the full list of collections available in our Strapi CMS

It also adds the link to this section in the TOC of the collections.md file.

Resolves Issue#42

[LauraLangdon]

Thanks for seeing this through, @ll-zerr!