Updates CONTRIBUTING guide

[ispivey]

The goals of this update are:

• Update documentation about Snippets to reflect that TypeScript files are the source of truth, and they must be transpiled.
• Generally, update anything that references JavaScript to reference TypeScript as well or instead.
• More clearly separate reference content (e.g. "these are the expected metadata fields in metadata.toml") from tutorial content (e.g. "clone X repo, make a PR to Y repo").
• Explain how boilerplate templates work more clearly.
• Polish up any typos, links, etc.

You can view a Markdown-rendered version of the final result here: https://github.com/cloudflare/template-registry/blob/c7abbfbe3c7433a506911cf8d195c2739f91a6fb/CONTRIBUTING.md

[ispivey]

Got review async from @koeninger .

I'll merge this and make some follow-up improvements to emphasize that boilerplates can and should be written in any language, not just JavaScript or TypeScript.