[INFRA] expand submenus on landing page automatically

[sappelhoff]

Let's see if this closes #925.

[sappelhoff]

• old: https://bids-specification.readthedocs.io/en/latest/

• new (this PR): https://bids-specification--937.org.readthedocs.build/en/937/

I like it better this way. --> Although we have to note that (1) for mobile, everything stays as it is currently, and (2) we cannot "collapse" the main headings anymore. But personally, I am fine with both of these points.

WDYT @tsalo? Is this what you had in mind?

[Remi-Gau]

I'd fine with this. 😄

[tsalo]

Love this. Thank you!