add initial draft of MaskWorkspace docs

[walshmm]

Description of work

Sliced the docs portion out of this pr: #40302

This PR seeks to add at least an initial draft doc page describing MaskWorkspace and its inner workings.
This will hopefully provide developers with a better intuition for how to use it and why it is the way it is.

To test:

Read the doc page and see if its helpful.

Reviewer

Your comments will be used as part of the gatekeeper process. Comment clearly on what you have checked and tested during your review. Provide an audit trail for any changes requested.

As per the review guidelines:

• Is the code of an acceptable quality? (Code standards/GUI standards)
• Has a thorough functional test been performed? Do the changes handle unexpected input/situations?
• Are appropriately scoped unit and/or system tests provided?
• Do the release notes conform to the guidelines and describe the changes appropriately?
• Has the relevant (user and developer) documentation been added/updated?
• If the PR author isn’t in the mantid-developers or mantid-contributors teams, add a review comment rerun ci to authorize/rerun the CI

Gatekeeper

As per the gatekeeping guidelines:

• Has a thorough first line review been conducted, including functional testing?
• At a high-level, is the code quality sufficient?
• Are the base, milestone and labels correct?

[github-actions]

👋 Hi, @walshmm,

Conflicts have been detected against the base branch. Please rebase your branch against the base branch.