Improve rerun_cpp readme & CMakeLists.txt

[Wumpf]

What

• Fixes Improve rerun_cpp readme.md and add it to the rerun_cpp_sdk.zip file #4117
• overhauled Readme.md for rerun_cpp
• everything except is now part of rerun_cpp zip bundle
• made rerun_cpp CMakeLists.txt smaller by splitting out the from-source rerun_c and the from-source arrow builds out
• rerun_c is now built as a built action, meaning it's part of the buildgraph of whatever generator you use - e.g. Ninja will build it now in parallel to your C files 🥳

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 demo.rerun.io (if applicable)
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

• PR Build Summary
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[Wumpf]

uh. needs windows fixing. Why am I not surprised

[Wumpf]

There was more wrong than just windows build. Had to kill the path of being in the repo and still setting RERUN_C_LIB cmake config variable. Realistically we don't need it and this makes things easier

[jleibs]

Overall looks great. Splitting out the arrow stuff makes the CMakeLists.txt much more approachable.

[jleibs]

Does this expose the variables necessary to implement https://github.com/kyle-figure/bazel-minimal-rerun/blob/main/third_party/rerun/patches/cmake.patch ?

[Wumpf]

tbh I'm deeply confused (and troubled) about the visibility scope of cmake variables, so I don't know. Worst case this requires more patches then