Rollup merge of #126039 - dpaoliello:arm64ecbuild, r=davidtwco

Promote `arm64ec-pc-windows-msvc` to tier 2

MCP: <rust-lang/compiler-team#746>

* Update platform support docs
* Add `arm64ec-pc-windows-msvc` as a target to the existing AArch64 Windows build in CI.
* Fix docs build break.
* Add `arm64ec-pc-windows-msvc` to build manifest.

CI build (succeeded, but upload to S3 failed): <https://github.com/rust-lang/rust/actions/runs/9388227822/job/25853013083?pr=126039>

Author: GuillaumeGomez