ENH: Add T1 and FLASH BEM to website

[larsoner]

Before merging …

• [x] Changelog has been updated (docs/source/changes.md)

1. Refactor BEM report plotting as helper
2. Add it to the BEM surfaces step (will be replaced in the forward step e.g. if the BEM surfaces don't need to be made / already exist, which should be fine)
3. Add back T1_BEM and FLASH_BEM to website now that their reports exist

4. Add some ignores to mkdocs using mkdocs-exclude plugin to avoid some of the cruft visible here:

   

[larsoner]

Okay it finally works after some cycles needed to properly DRY our CircleCI config a bit:

• ds000248_FLASH_BEM
• ds000248_T1_BEM

Just pushed a [ci skip] commit to fix the redirector, too, but we won't be able to see that work until it lands in main.

[larsoner]

@hoechenberger feel free to review and merge if you're happy

[hoechenberger]

Thanks @larsoner!