Add Instructions for Remotely Mounting Archive on macOS

Description of work

Summary of work

Added some more information to the developer documentation outlining how to mount the archive when working through the VPN on macOS.

Purpose of work

I only today discovered that you could actually do this, so adding it to the dev docs as it's something I've been asked about before.

Further detail of work

In addition to the new instructions, also improve the dodgy formatting.

To test:

Work remotely.
Build the dev docs
Follow the new instructions and check the archive mounts properly.

This does not require release notes because it only changes developer documentation. No user-facing changes.

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?
If there are changes in the release notes then do they describe the changes appropriately?
Do the release notes conform to the release notes guide?

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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mantidproject / mantid