Add CITATION.cff + README cross-link to product page

[nathanschram]

CITATION.cff

Add a CITATION.cff file so GitHub renders a "Cite this repository" button. Example:

cff-version: 1.2.0
message: "If you use this software, please cite it as below."
title: "Contextdocs"
authors:
  - family-names: Schram
    given-names: Nathan
type: software
repository-code: "https://github.com/littlebearapps/contextdocs"
url: "https://littlebearapps.com/builds/contextdocs/"
license: MIT

README cross-link

Ensure the README prominently links to the littlebearapps.com product page with contextual anchor text describing what the page offers (help docs, roadmap, community).

Example:

Full documentation, roadmap, and community →

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Part of the GitHub integration SEO/GEO improvements tracked in littlebearapps/littlebearapps.com#132.