Better 404 pages/redirection for static index meta-databases

[ml-evs]

It would be good to have a better default 404 template, such that when someone visits e.g. http://providers.optimade.org/index-metadbs/tcod, they get a link to the appropriate links endpoint. This has come up a few times in tutorials and other issues (#75, Materials-Consortia/providers-dashboard#60 and #74 (comment))

[ml-evs]

The other option would be to redirect all "raw" base URLs to the appropriate links endpoint.

[CasperWA]

The other option would be to redirect all "raw" base URLs to the appropriate links endpoint.

I see you've already implemented this in #77, but I'm not sure this is the "correct" way to handle this. However, it is convenient. Personally, I'd prefer a web page response, as I may be a confused client going to the base URL and suddenly be redirected to a functional entry endpoint - it seems off, but it's very practical.

[ml-evs]

We can change my redirects to instead serve a custom 404 page (for just the providers base URLs), but what info should be on it? We can't use a templating engine or anything, so I don't think we can provide a link to the links endpoint, or serve any provider-specific data as we do in the OPT landing page (unless there are some HTML-only hacks for relative redirects/getting the current requested URL).

Other option of generating a 404 page per provider and committing in this repo seems like too large a burden to me.

[CasperWA]

I would just add some hard-coded "common shortcomings/mistakes" list with a mention of "did you remember to add the major version and endpoint to the path?" or something?

[ml-evs]

Have updated my PR with something along these lines.

[ml-evs]

Closed by #77