[DOC] Update developer documentation

[MatthewMiddlehurst]

Updates the developer guide pages and a couple of others to better reflect the current state of the project. Could use a proof read.

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I have added the following labels to this PR based on the title: [ $\color{#F3B9F8}{\textsf{documentation}}$ ].

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[TonyBagnall]

LGTM, nice job.