CI: Add ability to build specific chapters or sections

[AnirudhDagar]

It will be useful to add a d2l bot command to specify running only a few chapters or sections. For example, a change in #2121 needs to be tested against chapter 16 and also sections from 15.8 onwards.

Pitch

Commenting something like the following on a PR:

@d2l-bot please build chapter_natural-language-processing-pretraining and chapter_natural-language-processing-applications.

should rerun CI for all the sections in these chapters. There is no need to rebuild from scratch for all the chapters, but rebuilding these would definitely help in ensuring the validity of a change while saving a lot of resources.

cc @astonzhang

[astonzhang]

This feature will be helpful for changes that may affect a subset of notebooks!