feat: add base prompt txt page (#20376)

* feat: add base prompt txt page

* feat: add sidebar links for prompt

---------

Co-authored-by: [email protected] <[email protected]>

Author: deloreyj

src/content/docs/agents/platform/prompt.txt.mdx

@@ -0,0 +1,9 @@
+---
+pcx_content_type: navigation
+title: prompt.txt
+external_link: /workers/prompt.txt
+sidebar:
+  order: 99
+head: []
+description: Provide context to your AI models & tools when building on Cloudflare.
+---