[Doc 404] benchmark branch is missing according to docs

[farzinlize]

hello and thank you all for your great works at flatbuffers

I was reading the benchmark page at flatbuffers.dev which I also found it at docs/source/benchmarks.md in this repository.

It is written at the bottom of this article that the git repository should have benchmark branch but I was only able to find ./benchmarks directory in the master branch. Plus that, the directory also misses other libraries that were tested against flatbuffer, like RapidJSON or Protobuf etc. (this seems logical because the master branch should not include those dependencies for testing purposes, I guess)

I was wondering if I can find the exact code to generate the whole benchmark by myself on my own machine, to test the flatbuffers against those other options that were included in the docs.

Anyway the document needs update if benchmark branch is removed or replaced elsewhere, to prevent confusion.