Update page generator to support linking to assets correctly

[Montoya]

See this comment:

For example this link - https://metamask.github.io/SIPs/assets/sip-9/example-snap/, it exists in the repository but 404s on the page, we would either have to update the SIP to point to specific files instead of directories or update the page generator

This issue should be resolved so we can use the website for canonical links to SIPs.

[FrederikBolding]

@ritave Static files are available on the website, e.g. https://metamask.github.io/SIPs/assets/sip-9/example-snap/snap.manifest.json - Is the problem that we want to show a directory structure for dirs?

[ritave]

@FrederikBolding yes, it's fine if it just redirects to GitHub directory view instead.