docs: Add documentation on Spack builds

[stephenswat]

This commit adds a new section to the "getting started" section of the documentation detailing how Spack can be used to install the dependencies of ACTS.

[paulgessinger]

Wondering if we should keep the part in getting started relatively short and link to a separate doc page with more info like how environments work.

There we could also add documentation on the build caches.

Invalidated by push of fb96e05

[stephenswat]

I've moved the Spack section to a new "Miscellaneous topics" section and linked to it.

[andiwand]

What I am missing a bit from the documentation is how do we sync spack if it gets outdated? If I understand correctly from a previous discussion they keep all the descriptions in the main repository and we would have to update there? Is there another way? Can we host the recipe somewhere?

[acts-project-service]

✅ Athena integration test results [d7392a5]

✅ All tests successful

status	job	report
🟢	report_pull_request
🟢	run_art_test: test_data18_13TeV_1000evt
🟢	run_art_test: test_ttbarPU40_reco
🟢	test_ActsDumpGeometryIdentifiers
🟢	test_ActsCheckObjectCountsCachedWorkflow
🟢	test_ActsCheckObjectCountsWorkflow
🟢	test_ActsEFTrackFit
🟢	test_ActsPersistifySeeds
🟢	test_ActsBenchmarkWithSpot
🟢	test_ActsAnalogueClustering
🟢	test_ActsWorkflowHeavyIons
🟢	test_ActsWorkflowFastTracking
🟢	test_ActsWorkflowCached
🟢	test_ActsWorkflow
🟢	test_ActsValidateAmbiguityResolution
🟢	test_ActsValidateResolvedTracks
🟢	test_ActsValidateTracks
🟢	test_ActsValidateActsCoreSpacePoints
🟢	test_ActsValidateActsSpacePoints
🟢	test_ActsValidateSeeds
🟢	test_ActsValidateOrthogonalSeeds
🟢	test_ActsValidateClusters
🟢	test_ActsPersistifyEDM
🟢	test_ActsGSFRefitting
🟢	test_ActsKfRefitting
🟢	test_ActsExtrapolationAlgTest
🟢	test_ActsITkTest
🟢	run_workflow_tests_run4_mc
🟢	run_workflow_tests_run2_mc
🟢	run_workflow_tests_run2_data
🟢	run_workflow_tests_run3_mc
🟢	run_workflow_tests_run3_data
🟢	run_unit_tests