CI build-speed optimisations

[firthm01]

• Replace cache even if cache hit (this resolves issues where the cache didn't include all vcpkg dependencies and some had to be built)
• Perform vcpkg dependency building and caching as a separate job (cache is still saved if project build fails)

[firthm01]

• do packaging docs out-of-date check first

[firthm01]

Second comment now resolved.
First comment could be achieved by running the current job up to (and including) cmake "configure" - this will build vcpkg deps. Do not do build/test/package steps. Post Cache step will store vcpkg builds. Second job should pick up as it is currently.
Note that for "good" code, this isn't really a speed optimisation - in fact probably a little slower. But it will be substantially faster when we're working with bad code and failing builds.