Improve 'before you begin' section in code snippets user docs

[Melissa Henderson]

Each code snippets page has a Before You Begin section that contains multiple expands for signing up with/setting up Rafiki.money. However, after going through the steps once, you don't need to do them again. Instead of having the same set of expands on every page, let's create a stand-alone page and link to the content from the code snippets pages.

• Create a "Before you begin" page in the Code Snippets section that contains the content of the original 4 partial files. Delete the partial files.
• To ensure we use the same lang string and link(s) on each code snippet page, create the new 'before you begin' content within a partial and add to each page
• Update each code snippet page by changing the partials 'import' to call the new partial file

[Melissa Henderson]

This is complete from a docs standpoint, but I'm having problems with PR #377. When that's resolved, the updates in this issue will be available.