Consolidate developer documentation from various sources, ensuring accuracy

[lukehesluke]

So developer documentation is currently split between:

• Current live docs (developer.openactive.io)
• Sam's new docs (https://app.gitbook.com/o/-LJTExWaXZFqcFJ_lZwF/s/IxxB3mlTS88NK1MDhi40/)
  • A rough version of this that predates Sam S's docs (https://app.gitbook.com/o/-LJTExWaXZFqcFJ_lZwF/s/-Mc9auy4nyx2KKbtF5To/)
  • Note Sam's docs are missing "estimated time/complexity" for each of the sections, which is important as it helps developers who are estimating the work figure out what's involved
• An attempt to simplify the explanation of OpenActive can be found at (https://app.gitbook.com/o/-LJTExWaXZFqcFJ_lZwF/s/-MSchL_Oqi4ju0Lk_6RL/) - note this is likely to be accurate in many places as it has not been reviewed at all