Document how to add plan to dodal

[dan-fernandes]

There is currently no documentation on how to add plans to Dodal, or equivalent repos (such as sm-bluesky).

In order for BlueAPI to pick up a plan, it must be added to the module's __init__.py. This requirement is not necessarily obvious.

Acceptance Criteria

• A page exists documenting how to add a plan to Dodal

[Relm-Arrowny]

I thought you could just use the module's direct import path? Does it really need to be defined in init.py?

[oliwenmandiamond]

No, plans must be added to __all__ for blueapi to detect a plan which is only done if we add it to via __init__.py file
https://diamondlightsource.github.io/blueapi/main/how-to/write-plans.html#detection

[Relm-Arrowny]

No, plans must be added to __all__ for blueapi to detect a plan which is only done if we add it to via __init__.py file https://diamondlightsource.github.io/blueapi/main/how-to/write-plans.html#detection

If there is an [__all__ dunder](https://docs.python.org/3/tutorial/modules.html#importing-from-a-package) present in the module, blueapi will read that and import anything within that qualifies as a plan, per its type annotations. If not it will read everything in the module that hasn’t been imported, for example it will ignore a plan imported from another module.

This imply it will import plan from a module that does not have all:
https://github.com/DiamondLightSource/blueapi/blob/18e04caaa36aeb2900f927bd8fa89b25ad024e30/src/blueapi/core/context.py#L263

I am 80% sure I used to import just 1 module for blueapi by doing something like :

  - kind: planFunctions
          module: sm_bluesky.beamlines.i10.align_slits