docs: add a custom_edit_url for sdks and edge/proxy (#4985)

This change adds custom edit urls for all the external readmes (sdks,
edge, proxy) that we put in the docs. This makes it so that following
the "edit this document" link takes you to the correct location, instead
of to a file that doesn't exist in the main Unleash repo.

Important caveat: this will only work for repos where the readme is
called `README.md` (capitalization matters). Currently, this is all our
repos, but it's something to be aware of for later.

We don't currently record what the readme files are called directly in
the docs, so to know that for sure, we'd have to build an extra
integration to check and fix that automatically. It may be worth doing
in the future, but for now, I think it's fine. We could also just rename
the readme files if we find any issues.

website/remote-content/shared.js

@@ -119,6 +119,7 @@ module.exports.modifyContent =
             content: `---
 title: ${data.sidebarName}
 slug: ${processedSlug}
+custom_edit_url: ${data.repoUrl}/edit/${data.branch}/README.md
 ---
 
 :::info Generated content