Rust docs cosmetics: add the "Made with Substrate, for Polkadot" to pallets

[sacha-l]

In an effort to make the pallet Rust docs have more of a uniform look, I'll start making PRs to update them so that they have the "Made with Substrate, for Polkadot" linking. But I'm opening this issue because I noticed it may be better to have this information after the heading. Cosmetics but .. the reason is that when you search for a pallet whose first piece of information is "Made with Substrate, for Polkadot", that's all you see and you don't have any useful info about it. For e.g. :

Rust doc will pick up the first line and use that. I think it's better to have the heading so that this info differentiates one pallet from another when searching. Please let me know if there's any disagreement.

For e.g. the top level crate doc would follow this structure:

[sacha-l]

cc @kianenigma