Add info + guide about Polkadot Fellowship Sub Treasury

[CrackTheCode016]

Addresses issue #6168

[filippoweb3]

Can we also provide the treasury and subtreasury addresses for Polkadot and Kusama with TabItem conditional rendering?

see https://assethub-polkadot.subscan.io/account/14xmwinmCEz6oRrFdczHKqHgWNMiCysE2KrA4jXXAAM1Eogk?tab=transfer

I wouldn't put the fellowship subtreasury info under the fellowship, but rather create a subtreasury section in the treasury page and list there all relevant subtresuries. This avoids fragementing info every time there is the need to add a subtreasury.

[filippoweb3]

this issue can be related to what we are discussing in here #6279

I propose a section about collectives, explain what collectives are and list the main ones. We can then add a subsection about subtreasuries.

[filippoweb3]

@CrackTheCode016 lmk what is needed to merge this one. I can see there is not sidebar integration for the new page.

[filippoweb3]

@CrackTheCode016 lmk the status of this PR and what is needed to go forward