Fix broken 1st-party docs.rs links, standardize version-linking

[EndilWayfare]

When reviewing the 0.7.x in-depth guides, I noticed that the "Template API docs" link was broken.

The dioxus_core::prelude module no longer exists, and Template struct is exported at root level.

While editing, I noticed some inconsistencies in how link urls to docs.rs are formed.

• Some of the legacy docs are still using ".../dioxus(-*)/latest", which is probably undesirable
• Some legacy docs reference 0.x.0, some reference 0.x.y (where y is the last patch version), and others reference 0.x.y (where y is neither 0 nor the last patch version).

Would probably be a good plan to decide a version-reference scheme to standardize on.

[EndilWayfare]

Oh, neat! docs.rs supports advanced semver URL redirections

https://docs.rs/about/redirections

Since the docs are already segregated in "0.x" directories, it seems sensible to me for all their docs links to use "~0.x".

[EndilWayfare]

I went ahead and fixed/standardized all docs.rs links (for 1st-party dioxus, at least).

Seems it was worth doing, since I found a few more outright-broken links

[EndilWayfare]

It seems that ".../0.x" works in addition to ".../~0.x" (even though the docs.rs redirection list doesn't include it), which makes sense because it matches cargo behavior.

I opted to stay with the "/~0.x" form, since it seems more explicit about "funny semver shenanigans are happening"

[EndilWayfare]

I made a solid effort to make sure that all the altered links are valid, but there are a lot of them