Add runtime config docs

[vdice]

Add topic guides for various real-world app scenarios that utilize runtime config. Per @bacongobbler's sage guidance, let's go with the following:

• Application secrets via external providers -> docs(topics): add external variable providers guide #203
• Connecting to a SQL database -> docs(topics): add sqlite provider tutorial #204
• Using a key/value store -> docs(topics): add key-value provider tutorial #213
• Using AI in your application