Revise Dev Portal Auth strategy page

JTBD:
- Add [this section](https://developer.konghq.com/dev-portal/self-service/#application-authentication-strategies) to [this page](https://developer.konghq.com/dev-portal/auth-strategies/), and remove from existing doc
- Might be a repeat of the first bullet, but move [this section](https://developer.konghq.com/dev-portal/security-settings/#authentication-strategy-and-creating-api-keys) to the [application auth page](https://developer.konghq.com/dev-portal/auth-strategies/) as well.
- I wrote this a long time ago, some of these might need to be includes. Regardless, the [application auth page](https://developer.konghq.com/dev-portal/auth-strategies/) should be the main hub for all things app auth, but some of these other pages like [self-service](https://developer.konghq.com/dev-portal/self-service/#application-authentication-strategies) might need an app auth section that briefly talks about it before linking off to the app auth page.

Start by reading the application auth page, the self-service section on application authentication strategies, and the security-settings section on authentication strategy and creating API keys. Consolidate the relevant material in the application auth page, remove duplicated content, and leave brief sections linking to the main hub where appropriate.

Written by the indexing model from the issue text.