* Update documentation and example for no_std (#8555)

* Update Wasmtime's tier stability documentation

Move some items between tiers and add a few misc items here and there.

* Update platform support documentation

Re-word lots of this since it was originally written, link to the tiers
of support page, and rewrite the section on `no_std`.

* Update the `min-platform` example with no_std

This commit updates the preexisting `min-platform` example to no longer
require Nightly Rust and instead use the `no_std` support now added to
Wasmtime. This involved:

* Change the build process to produce a staticlib which is then manually
  converted via `cc` into a shared library for the native Linux platform.
* Compile the modules outside of the embedding and only `deserialize`
  within the embedding.
* Update the `indexmap` dependency to pick up a bug fix required in
  `no_std` mode (apparently, it fails on indexmap@2.0.0 and passes at
  2.2.6, I didn't dig much further).

This commit additionally makes the `wasmtime-platform.h` header file
generated by the example a release artifact for Wasmtime itself. The
header itself is touched up a bit by configuring some more `cbindgen`
options as well.

* Fix clippy build

prtest:full

* Review comments

* Pass gc-sections to linking the library

* Add release notes for 21.0.0 (#8559)