WIP template on docs.rs for new crates in the queue

[zeenix]

When one publishes a new crate, it could take some time before docs.rs is able to build its docs for the first time and during that time, docs.rs/crate-name just says "no such crate". It would be great if it instead showed a template page that says that crate docs are currently in the build queue and will be available shortly.

[syphar]

This is totally possible to do, I would totally accept a PR solving this.

Since #1011 and #2533 we see the crates when we started building them, but not yet when they are queued.

That all being said: outside of exceptional situations like the last days, there isn't much time when crates are queued for longer than a minutes or perhaps hours.