Update core documentation

> [!IMPORTANT]
> This is a [roadmap](https://doomemacs.org/roadmap) issue and is used solely to announce and document progress (to make it easier for users to follow and maintainers to track). Please do not use it for discussion; use [our discussion board](https://doomemacs.org/discuss) instead.

The in-repo documentation is sorely out of date. This issue consolidates all open issues and pull requests for them into one. A total rewrite of our docs is in the works, which will resolve all the listed issues. Progress will be published here.

- https://github.com/doomemacs/core/issues/8548
- https://github.com/doomemacs/core/pull/8527
- https://github.com/doomemacs/core/pull/8404
- https://github.com/doomemacs/core/pull/8341
- https://github.com/doomemacs/core/pull/8304
- https://github.com/doomemacs/core/pull/8301
- https://github.com/doomemacs/core/pull/8139
- https://github.com/doomemacs/core/pull/8122
- https://github.com/doomemacs/core/issues/8112
- https://github.com/doomemacs/core/issues/7686
- https://github.com/doomemacs/core/issues/7631
- https://github.com/doomemacs/core/pull/7570
- https://github.com/doomemacs/core/pull/7406
- https://github.com/doomemacs/core/pull/7371
- https://github.com/doomemacs/core/issues/7252
- https://github.com/doomemacs/core/pull/7216
- https://github.com/doomemacs/core/issues/7142
- https://github.com/doomemacs/core/issues/7135
- https://github.com/doomemacs/core/issues/7056
- https://github.com/doomemacs/core/issues/5694
- https://github.com/doomemacs/core/issues/5663
- https://github.com/doomemacs/core/issues/4128
- https://github.com/doomemacs/core/issues/3670
- https://github.com/doomemacs/core/issues/2163
- https://github.com/doomemacs/core/issues/372