example readmes for C++ and C (#4076)

### What

* part of  #3962
* Readme for C++ & C examples

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested [demo.rerun.io](https://demo.rerun.io/pr/4076) (if
applicable)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG

- [PR Build Summary](https://build.rerun.io/pr/4076)
- [Docs
preview](https://rerun.io/preview/35a6473f5e8698543c740b95516fed5a4222ee4c/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/35a6473f5e8698543c740b95516fed5a4222ee4c/examples)
<!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://ref.rerun.io/dev/bench/)
- [Wasm size tracking](https://ref.rerun.io/dev/sizes/)

examples/c/README.md

@@ -1 +1,8 @@
-The Rerun C SDK is in its infancy, and not ready for production.
+# Rerun C API
+
+The Rerun C library provides a minimalistic C interface that encapsulates the shared building blocks of all Rerun SDKs.
+It's a key dependency of the Rerun C++ SDK, serving as the primary language interface into the Rust codebase.
+
+⚠️ It currently serves *exclusively* this language binding purpose.
+It is not a full SDK yet as there's no utilities for serializing data for logging any of the built-in types.
+As of now it can only log raw Arrow IPC messages.

examples/cpp/README.md

@@ -1,18 +1,12 @@
 # Rerun C++ Examples
-The simplest example is [`minimal`](minimal/main.cpp). You may want to start there!
+The simplest example is [`minimal`](minimal/main.cpp). You may want to start there
+using the accompanying [`C++ Quick Start`](https://www.rerun.io/docs/getting-started/cpp?speculative-link) guide.
 
-TODO(#2919): This example is not ready yet.
+## Build all examples
+The CMake target `examples` is a convenient alias for building all CMake examples in one go.
 
-Read more about our examples at <https://www.rerun.io/examples>.
-
-## Setup
-TODO(#2919): Document.
-
-## Dependencies
-TODO(#2919): Document.
-
-## Running the examples
-TODO(#2919): Document.
+You can use `pixi run cpp-build-examples` to invoke it within the repository's Pixi environment.
+After that, you can run individual examples from `./build/examples/cpp/` (e.g. `./build/examples/cpp/dna/example_dna`).
 
 ## Contributions welcome
 Feel free to open a PR to add a new example!