[libc++] The "Exporting new symbols from the library" paragraph in `Contributing.rst` needs updating

We're currently exclusively taking about buildkite there, with examples that aren't applicable anymore. This should be updated to also say where to get them from GitHub and the examples should be updated so people don't get confused.

Open the "Exporting new symbols from the library" paragraph in `Contributing.rst` and review its current Buildkite instructions and examples. Check the repository's GitHub workflow for obtaining exported symbols, then update the paragraph and examples so they describe both sources accurately; confirm the rendered documentation is clear.

Written by the indexing model from the issue text.