Rename the hero "Submit" button and say it opens GitHub

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Problem

The home page hero has a button labeled only "Submit" (components/LibraryExplorer.tsx). It opens a new GitHub issue in a new tab. Users do not know what it does until they click it and land on GitHub.

Acceptance criteria

• The label says what happens, for example "Suggest a library".
• The button shows it leaves the site, with an external link icon or a short note that it opens GitHub.
• Link to a prefilled issue template for library requests, or to the "Request a library" docs page first.