feat(content): add monetization/premium apps page

[shiftinv]

Description

This adds an introductory page about premium applications/monetization on Discord. See https://github.com/DisnakeDev/disnake/pull/1113 and https://discord.com/developers/docs/monetization/overview for references.

The library implementation isn't released yet, so I'll keep this PR as a draft until then.

---

preview: https://feature-monetization.disnake-guide.pages.dev/popular-topics/monetization