Clarify docs on GH release install & C++ source build, remove reundant rerun_cpp_sdk artifact

[Wumpf]

What

Minor doc cleanup plus removing redundant c++ sdk zip from future artifacts:

The less verbose of these will no longer show up in the next release:

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

• PR Build Summary
• Recent benchmark results
• Wasm size tracking

To run all checks from main, comment on the PR with @rerun-bot full-check.

[github-actions]

Deployed docs

Commit	Link
0883042	https://landing-aubcgg0ag-rerun.vercel.app/docs