Add "Miscellaneous" section to developer documention

## Remaining areas to document

From https://github.com/MetOffice/CSET/pull/1638#issuecomment-3241945549
> Rose bunch, which is used for the parallelism here, handles putting each task's logs into a separate file, so they won't be intermingled. The main job.out and job.err files contain the currently running jobs and point to the log files for any failed jobs, so debugging is still possible, though it does require an extra click to see exactly what has failed.
>
> To ensure this is discoverable it will be added to the documentation in part 7 of this patch series.

From https://github.com/MetOffice/CSET/pull/1639#issue-3356342572
> The documentation for adding a new diagnostic/recipe will need updating to reflect include files no longer being a thing.

From https://github.com/MetOffice/CSET/pull/1639#pullrequestreview-3172746180
> It should be noted in the documentation updates that the `run_cset_recipe` app folder needs to be deleted in the users `cset_workflow/app` directory as it is not tracked by GitHub and will cause problems for opening the GUI.

`base_model` cube attribute.

Recipe file naming convention.

_Originally posted by @jfrost-mo in https://github.com/MetOffice/CSET/issues/1688#issuecomment-3266319468_

Start by locating the developer documentation, including the section covering new diagnostics or recipes and the referenced part 7 updates. Document Rose bunch log behavior, the removal of include files, deletion of the user's cset_workflow/app/run_cset_recipe folder, the base_model cube attribute, and recipe naming; done means each listed area is discoverable and accurate.

Written by the indexing model from the issue text.