keep auto-generated content in docs out of VCS

[boneskull]

Our documentation has automatically generated sections. The content in these sections is part of the doc build and should not be in VCS.

<!-- AUTO-GENERATED-CONTENT:START (toc:maxdepth=2&bullets=-) -->
(stuff)
<!-- AUTO-GENERATED-CONTENT:END -->

Should not be modified when running npm start docs.

Instead, the build should be run against a transient copy of the file. That means:

1. Copy docs/index.md to a temp location
2. Run the preprocess script against the temp copy
3. Ensure Eleventy generates documentation from the temp copy
4. Remove the temp copy

Take care not to deploy the temp copy itself; Eleventy should deploy HTML only, and no markdown.

(This also applies to tutorials found in the API docs, which are in incoming pull request #3655; don't worry about these for now.)