Fix "Edit this page" button (leads to 404)

[maxheld83]

The "Edit this page" button currently leads to a 404, as first reported in #391

The web UI currently gives this for https://zero-to-nix.com/start/nix-develop/, for example:

https://github.com/DeterminateSystems/zero-to-nix/edit/main/src/content/start/nix-develop/.mdx

The Correct URL would be:

https://github.com/DeterminateSystems/zero-to-nix/edit/main/src/content/start/3.nix-develop.mdx

So, it seems like the filename is bad.

[maxheld83]

I think this is the bug:

zero-to-nix/src/components/FeedbackBar.astro

Lines 9 to 14 in cbaf514

	const filename = Astro.url.pathname;
	const issueTitle = `Issue with page \`${filename}\``;
	const issueBody = `...Replace me with your issue description...`;
	const issueUrlParams = `title=${issueTitle}&body=${issueBody}`;
	const issueUrl = `${githubUrl}/issues/new?${encodeURI(issueUrlParams)}`;
	const editUrl = `${githubUrl}/edit/main/src/content${filename}.mdx`;

filename is the url path (which doesn't have the starting 3.).

I'm not sure what the idiomatic/intended file/url naming scheme is here, so I'll leave this for someone else to fix who knows more.