[WIP] Reduce total CI duration

[elle-j]

Description

Current changes:

Change	Now	Before
Per-target build sub-workflows	Split umbrella reusable-build.yml into reusable-build-{linux,macos,windows,wasm}.yml. Consumers can wait on a specific target and start earlier instead of blocking on the whole matrix. E.g. our longer e2e tests now start earlier.	Getting a reusable build depended on all builds completing. Even if e.g. a macOS build is cached, the waiting workflow with macOS runners could be queued for a long time just to get the status and cache of the other workflow, since macOS runners are more scarce than e.g. ubuntu.
Bottleneck macOS Intel only running on main	macOS Intel runners are 2-3× slower than macOS ARM for the contract compilation tests. This build is now excluded from the matrix for PR runs, and only included for pushes to main.	Ran on each PR, being the bottleneck.
Hardhat unblocked from differential tests	Starts as soon as the required test job succeeds, using the reusable Linux build.	Started after the differential tests succeeded in order to reuse the linux build.
Wasm tests use cached artifacts	test-wasm.yml uses the reusable Wasm build.	Ran its own duplicate build.
Faster LLVM downloads	get-llvm/action.yml uses more efficient version detection. Building resolc now takes ~2min (~6 min on macOS intel).	Building resolc took ~8-13min (~26 min on macOS intel).
Conditionally skip workflows	Most workflows are skipped for doc-only updates. Tooling-only updates trigger only the respective tooling test (Hardhat), apart from the required test job.	Conditionally skipped for md-only changes. Tooling-only changes also ran e.g. test-wasm.yml and differential-tests.yml.