Document toc.md and toc.html and make toc.md a reserved file

Problem

In normal circumstances mdbook will generate a file called toc.html that will contain the Table of Contents`. It is used as the TOC for people (and bots) who don't run JavaScript.

If the user creates a file called src/toc.md and references it in SUMMARY.md then mdbook will use it to generated the toc.html file replacing the normally generated one.

Proposed Solution

IMHO this should be avoided.

1. If there is a an toc.md file the build process should stop with an error toc.md is reserved for internal use.
2. Regardless of 1. being implemented or not, the behaviour should be described in the documentation so people can search for either toc.html , toc.md or toc and find it.

Notes

See also #3047

Start by reading the build handling for src/toc.md and the SUMMARY.md entry that can cause toc.html to be replaced. Review issue #3047 and the existing documentation for toc.html, toc.md, and toc, then add the reserved-file error and document the behavior. Done means the conflict is rejected and users can find the behavior in the documentation.