Add instructions for creating custom mantid-developer package

[jhaigh0]

Description of work

Add instructions for creating custom mantid-developer package to Packaging.rst

There is no associated issue.

To test:

Follow instructions to test. Use conda list in your new environment to check the correct changes were made.

Docs build with good formatting.

This does not require release notes because change to dev docs

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Reviewer

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Code Review

• Is the code of an acceptable quality?
• Does the code conform to the coding standards?
• Are the unit tests small and test the class in isolation?
• If there is GUI work does it follow the GUI standards?
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Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?
• Do the changes handle unexpected situations, e.g. bad input?
• Has the relevant (user and developer) documentation been added/updated?

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[jclarkeSTFC]

Looks good, just a couple of typos. It will be very useful to have these instructions in the docs.