[DOC] Update website developer guide

[MatthewMiddlehurst]

Follow up from #1212

Updates the developer_guide.md page and the pages under the developer_guide/ directory.

Removes the git_workflow and add_dataset pages, which seem unnecessary to me. Will add them back if people disagree.

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[TonyBagnall]

the buttons from here dont seem to do anything
https://aeon-toolkit--1259.org.readthedocs.build/en/1259/developer_guide.html

[MatthewMiddlehurst]

dang it. Was gonna go through and check all the links but beat me to it. Will update 🙂