docs(dev/AppConfig): re-organize, expand, update, and improve clarity

[joshtrichards]

☑️ Resolves

• Re-organized for more logical flow
• Fixed some typos
• Added clarifications for consuming the APIs (in particular between the two inter-related IAppConfig APIs)
• Added missing method(s)
• Expanded coverage of typed config values and specifically:
  • migrating (reading / get*)
  • migrating (writing / set*)
• Expanded coverage of lazy loading and specifically:
  • "Choosing what should be lazy"
  • "Reading behavior with lazy parameter"
• Updated code examples

Related/follow-up items:

• The Basics->Config chapter could probably be better unified:
  • https://docs.nextcloud.com/server/latest/developer_manual/basics/storage/configuration.html
    • If kept separate, it also could use an update to use IAppConfig probably too - e.g. https://docs.nextcloud.com/server/latest/developer_manual/basics/storage/configuration.html#app-values
• UserConfig chapter could use a similar pass:
  • https://docs.nextcloud.com/server/latest/developer_manual/digging_deeper/config/userconfig.html

🖼️ Screenshots