Cookbook files should define the custom_edit_url frontmatter

[ElliotFriend]

Following on from #1541 and stellar/stellar-docs#840

If these cookbook docs remain here, each one will need to define its own custom_edit_url frontmatter value, so the "Edit this page" button on the bottom of each one points people in the right direction. Currently, clicking on that button from any of these pages results in a 404 error, since the file is no longer in the stellar-docs repo.

For example, the frontmatter of the "Payments and Assets" page should be adapted to look something like:

---
title: Payments and Assets
hide_table_of_contents: true
description: Send XLM, stellar classic, or a soroban asset using the Stellar CLI
custom_edit_url: https://github.com/stellar/stellar-cli/edit/main/cookbook/payments-and-assets.mdx
---

To send payments and work with assets ...

Docusaurus info about this frontmatter value: https://docusaurus.io/docs/api/plugins/@docusaurus/plugin-content-docs#custom_edit_url.