[Feature][Docs] Add "collapsible" (true/false) attribute to section config

[hkosova]

Problem description

Section config in docs.yml currently supports the collapsed attribute which is very useful to manage large TOCs.

It would be great to also have the collapsible attribute so that we could create collapsible sections that are expanded by default.

Why would it be useful?

• More consistent UX for sections - even if some sections are expanded by default, they can still be collapsed.
• Feature parity with other documentation tools (e.g. Docusaurus).

Describe the solution (optional)

• Sections have a boolean collapsible attribute.
• Default value can be false to keep the current default UX.

Additional context