[master < T350] Add graphml docs

[ind1xa]

Description

Added documentation for graphml import and export functions.

Pull request type

• [x] New documentation page

Related PRs and issues

PR this doc page is related to: https://github.com/memgraph/mage/pull/327

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