Conda developer docs

[robertapplin]

Description of work
This PR adds a developer documentation page on how the Conda package manager can be used in Mantid. It provides tips on debugging dependency issues in our pipelines, and recommendations on how they should be fixed. It also includes a policy on our attitude towards using PyPi packages in Mantid (we strongly recommend that we do not).

This PR doesn't add all the Conda documentation we probably need. I have left some checklist points in the attached issue which would benefit from being written from other team members with experience in these areas. But I think this document makes a good start and can be added to in the future.

To test:
Code review. Build dev-docs target and make sure the links are working.

Part of #35835

This does not require release notes because it is dev docs

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• Do changes function as described? Add comments below that describe the tests performed?
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[cailafinn]

Looks good, and nice to have this in. Just a few typo/style/etc. comments

[robertapplin]

Thanks @ConorMFinn , its ready for another look

changes made

[cailafinn]

Looks good to me now. Nice work