[master < schema] Write schema docs

[mpintaric55334]

Description

Please briefly explain the changes you made here.

Write docs for schema module.

Pull request type

Please delete options that are not relevant and check the ones that are.

• [x] New documentation page

Related PRs and issues

PR this doc page is related to: https://github.com/memgraph/mage/pull/341

Closes (link to issue):

Checklist:

• [x] I checked all content with Grammarly
• [x] I performed a self-review of my code
• [x] I made corresponding changes to the rest of the documentation
• [x] The build passes locally
• [x] My changes generate no new warnings or errors