Add logos to the published documentation.

[abizjak]

Task description

Add documentation metadata to the published crates, in particular we should set the logo and issue tracker urls, see https://doc.rust-lang.org/rustdoc/the-doc-attribute.html#at-the-crate-level

Sub-tasks

• [ ] Add favicon to std, std-derive, contracts-common
• [ ] Add logo to std, std-derive, contracts-common
• [ ] Add issue tracker urls for all of those

The logo and icon are attached (they don't have a background and the foreground is a white logo, so they might appear as blanks below)