Add a How to contribute static button in polkadot-sdk docs

[rzadp]

The polkadot-sdk docs have this static source button on each page, that takes you to another (hidden) docs page which displays the Rust source code of the given page.

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This is a default mechanic of Rust documentation.

It was wished for, that we have another static button, that would take you to github source code - so the documentation consumer could contribute.