docs: add best code snippet managers comparison page

[antonreshetov]

Adds a category comparison page /compare/best-code-snippet-managers targeting high-intent listicle queries ("best code snippet managers", "code snippet manager/tool/library", "snippet software").

• Honest listicle comparing massCode, SnippetsLab, Pieces, Cacher, Lepton, GitHub Gist, Raycast Snippets, and VS Code snippets on license, platforms, storage, account, and scope.
• At-a-glance table, per-tool breakdown, how-to-choose, by-use-case recommendations, and FAQ for SEO/AI snippets.
• All competitor facts verified against official sources at write time (SnippetsLab now free; Pieces on-device; Lepton flagged as unmaintained; Cacher active).
• Registered in the compare sidebar (config.mts) and compare/index.md.