Spell out which plugins are supported by each platform and which ones we're missing

[essential-randomness]

For example:

Bluesky

• Links ✅
• Spoilers ⚠️ (with adapter)
• Colored Highlight ❌

Leaflet

• Links ✅
• Spoilers ⚠️ (with adapter)
• Colored Highlight 🫥 (missing)

Platforms we need it for:

• Bluesky
• Leaflet
• Tumblr
• Twitter
• Mastodon