Add runtime config docs

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

[x] Application secrets via external providers -> https://github.com/spinkube/documentation/pull/203
[x] Connecting to a SQL database -> https://github.com/spinkube/documentation/pull/204
[x] Using a key/value store -> https://github.com/spinkube/documentation/pull/213
[ ] Using AI in your application

spinkube / documentation