doc: Document `build.commands` working directory (#11218)
I just got bit by this trying to do
```yaml
build:
commands:
- cd docs
- myst build --html
```
After the `cd docs`, the next command is reset to the main repo root
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:books: Documentation previews :books:
- User's documentation (`docs`):
https://docs--11218.org.readthedocs.build/en/11218/
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- Developer's documentation (`dev`):
https://dev--11218.org.readthedocs.build/en/11218/
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