Clean up documentation page sidebar

[jklaise]

As the number of examples has exploded, the organisation of them in the documentation page sidebar without sections has become unwieldy. We should do something similar to the Alibi Detect docs.

Important: we should make our best efforts to not just remove the old link paths to avoid "breaking the internet". This should be ok as looking at Alibi Detect, the example notebook paths all remain unchanged, they're just grouped into the new "example grouping" pages.

[ascillitoe]

Totally agree!

As you say, this was done for detect by moving examples (and overview pages) under their relevant cd/, od/, ad/ folders, and then adding separate toctree's to index.md:

```{toctree}
:caption: Outlier Detection
:maxdepth: 2

od/methods
od/examples

:caption: Drift Detection
:maxdepth: 2

cd/background
cd/methods
cd/examples

:caption: Adversarial Detection
:maxdepth: 2

ad/methods
ad/examples

Happy to do if/when you've decided on the groupings...

[ascillitoe]

Apologies, I misspoke above, as you said I didn't actually have to move any files around as just created links to the examples in the respective ad/ etc folders, for example:

:glob: true
:maxdepth: 1

../examples/ad_*

in oc/source/ad/examples.md. In other words, super easy!

[jklaise]

As part of this we should clean up the landing page as the highest-level heading is "Indices and tables" which becomes the title of the whole page. C.f. how it's handled in Alibi Detect.

[jklaise]

Closing as was done for the 0.7.0 release.