Add `singlerst` builder (single-file reStructuredText output)

[jdillard]

A useful addition would be a singlerst builder analogous to Sphinx’s core singlehtml builder. The goal is to produce a single concatenated reStructuredText file containing the entire documentation tree, preserving section structure but without splitting it into multiple files.

This would be useful in generating a single-file reST version of a docs project that can be used for llms-full.txt. The main reason being, converting reStructuredText to markdown for the llms-full.txt isn't a 1:1 relationship as markdown is more simple, so this could help maintain clarity.

An example of using the restbuilder for LLM text files can be seen in this PR: jdillard/sphinx-llms-txt#53

[jdillard]

I made an implementation PR #35 here and also demo using it in my project jdillard/sphinx-llms-txt#54