[chore] delete welcome.astro (used by extension, deprecated)

[sidvishnoi]

We can delete welcome.astro file as it was used by extension only, and now the extension has its own mechanism (interledger/web-monetization-extension#1238).

Caution

Delete it once users have migrated to new version of extension that doesn't use this welcome page. Around end of Q2 2026 seems reasonable.