docs(topics): add runtime config doc

[vdice]

Adds a Runtime Configuration topic guide.

Closes #199

[bacongobbler]

The content here looks great! However, I do wonder if it might make sense to break this document into separate topic guides:

• Handling application secrets
• Connecting to a SQL database
• Using a key/value store
• Using AI in your application

This gives the user the ability to search for a specific topic that would apply to their app. We can then use it as a "springboard" to discuss how to apply these topics to their application, including any trade-offs, examples, or alternative solutions.

If we wanted to document the specific schema of the runtime config, I might suggest placing that documentation under the reference guides. That would be a great place to put it - IMO most readers won't care about the runtime config schema moreso than the abilities it provides. That is, I believe they'd care more about how to add capability X to their application.

What do you think of this approach?

[bacongobbler]

See also https://www.spinkube.dev/docs/reference/spin-app/#spinappspecruntimeconfig

[vdice]

Thank you @bacongobbler, your vision for separate topic guides and then linking to the runtimeconfig spec reference where appropriate definitely seems the more helpful and reader-friendly way to go. On it! Will close those this PR but pull relevant bits into forthcoming topic guide PRs.