Improve Welcome Screen's Getting Started section

[abey79]

Currently, we only have links to the docs' getting started section for Python and Rust.

We need:

1. integrate the "getting started" (as markdown) inside the viewer
2. add support for C++ as well

[nikolausWest]

Ideal flow for the getting started section:

• Press the "Python/Rust/C++ Quickstart" button
• This runs code in the viewer that starts a new recording and logs a markdown document to the recording
• The code includes three sections:
  1. "Install the Rerun SDK" with minimal instructions on how to install the Rerun SDK (pip install rerun-sdk)
  2. "Stream data to this viewer" with copy short past-able code (for instance twisting cube) that streams data to the current viewer and recording. The init code should be dynamically set up so that it will connect and stream to the current viewer and recording. The code should include sleeps so that the user can see the data stream in (i.e. not all show up in one chunk)
  3. "How it works" with a basic rundown of how it works (although the code should be well documented)

It's important that this works both in the browser and the native viewer.