Fix sphinx warnings from RHEL 7 clean build

[NickDraper]

Fix the warnings found on the RHEL clean master build

No issue to close on this, PR only.

Release Notes

Not worth mentioning

To test

check the build output for warnings related to sphinx docs - Text Search for Warning

[mantid-builder]

Thanks for the changes. Here's a few things that we'll use to start the review process:

Main Reviewer

Please comment on the following:

Pull Request

• Does the title summarize the changes, i.e. it's not the automatically generated one?
• Is an issue link included/required, e.g. "Fixes #XXXX"? If there is an issue link do the changes look sensible and consistent with issue description?
• Are there adequate testing instructions?
• A link to the updated release notes has been provided or no release note updates were necessary?

Code Review

• Is the code of an acceptable quality?
  • Does the code conform to the coding standards? Is it well structured with small focussed classes/methods/functions?
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Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?
  • How do the changes handle unexpected situations, e.g. bad input?
• Has the relevant documentation been added/updated?
  • Is user-facing documentation written in a user-friendly manner?
  • Has developer documentation been updated if required?

---

• Does everything look good? Comment with the ship it emoji but don't merge.

Second Reviewer (@gatekeepers)

• Have adequate tests, both success & failure cases, been performed?,
• Has the code been reviewed?,
• Ready to go? Hit Merge!

[KarlPalmen]

This is not a fix of issue #13107 , but that is covered by issue #11386 .
I'll just test the documentation as described here.

[KarlPalmen]

I found plenty of warnings, but found that the built document files were OK, especially where changes were made (including links and headings). I do not have Latex so was not able to check the mathematical expression in Point and Space Group.

[KarlPalmen]