docs: add massCode vs GitHub Gist comparison page

[antonreshetov]

Adds /compare/github-gist targeting the "github gist alternative" query (already ranking ~position 2 with no dedicated page).

• Honest comparison: Gist's strengths (zero setup, sharing/embedding, real Git) vs its limits as a library (no folders/tags, weak private search, "secret" gists are not private, online-only).
• Migration angle: massCode imports public Gist URLs.
• All Gist facts verified against GitHub Docs at write time (secret gists not private; every gist is a Git repo; no native folders/tags; Nov 2025 secret-scanning of unlisted gists).
• Cross-linked with the new best-code-snippet-managers hub; registered in compare sidebar and index.