Modify PCP website to include new Homebrew Cask installation option

[tallpsmith]

Once all the new Darwin PRs are merged, we should update the PCP website to reference new Homebrew installation instructions.

Important PRs that need merging first:

• Fix macOS launchctl pmcd fork issue #2425
• darwin PMDA: Comprehensive metrics expansion #2442

Tasks

• Once PCP released, update the Homebrew Cask release details
• Update PCP website macOS installation documentation

[natoscott]

@tallpsmith merging those now - re the website comment, that would be greatly assisted by a PR or the pcp-website repo.