Add info about Polkadot Fellowship Sub Treasury

[DrW3RK]

Context: https://polkadot.polkassembly.io/referenda/832

[CrackTheCode016]

Assuming this content would go here: https://wiki.polkadot.network/docs/learn-polkadot-technical-fellowship

Then also mentioned where we talk about sub-treasures as an example? Might be good to have a list of them, or similar.

[DrW3RK]

A short description of the sub-treasuries in general would go on the treasury page differentiating it from OpenGov Treasury which is controlled by DOT holders. https://wiki.polkadot.network/docs/learn-polkadot-opengov-treasury

The goal and motivation of the fellowship subtreasury can go here https://wiki.polkadot.network/docs/learn-polkadot-technical-fellowship

The key contribution of this PR is the PJS guide which can be its own page.