Document why recipes all need to be in the same directory

One thing the documentation lacks is an explicit explanation of why recipes all need to be in the same directory (with the exception of core recipes). We really need to add that, since it's a significant difference from the way modules and themes work and trips people up if they don't understand the background or rationale behind the decision.

Draft

## Recipe location

Except for core recipes (located at `core/recipes`), all recipes on a given site must be in a single directory. This requirement is a key design feature of the recipe system and distinguishes it from, for example, the way that Drupal extensions (modules, themes) are managed and recognized on the file system.

Why?

A key reason is that recipes are ephemeral. They're not part of an architecture and so don't need to be carefully calibrated and balanced to interact with other elements. Instead, they're applied once, possibly reapplied, but they don't live on the site. There's no need, for example, for the complex set of overrides that apply in the extension case, where a contrib module overrides core one of the same name, only to be overridden in turn by a site-specific version.

Plus, Composer. It just makes sense to rely on it as a source of truth. Most Drupal site owners will never have to worry about where to put recipes. Composer will handle everything as needed.

If in doubt about where recipes are installed, look for the `installer-paths` section of your root `composer.json` file. If you used `drupal/recommended-project` to spin up your site, that line might look like:

```json
"recipes/{$name}": ["type:drupal-recipe"]
```

Meaning that, if other package types like modules and themes are installed to some subdirectory of `web`, composer will install recipes to a `recipes` directory one up from the web root. That's where any other recipes you might want to add will go too.

### How do you add your own custom recipes?

The first question should be, do you really need to? Again, it's important not to be distracted by a potentially misleading analogy with extensions. While there are indeed use cases for installing a custom module or theme on a given site, given recipes are something you apply once and are done with, a site-specific recipe that you want to keep around is pretty much a contradiction in terms.

That said, there will indeed be use cases for custom recipes. For example, there might be a set of recipes that you want to build out and use on a number of sites but not share with the world. You can maintain these in any of a number of ways:

* Commit your recipe to version control and add it to an internal Composer package. There are [several repository types that Composer supports](https://getcomposer.org/doc/05-repositories.md) which can accommodate this use case.
* Put your recipe wherever you like and then symlink it into the recipes directory.
* Commit your recipe directly to the recipes directory.

### What about dev environments and testing?

If you're developing recipes, you may have specific needs when it comes to linking in a recipe. For example, if you're using [DDEV Drupal Contrib](https://github.com/ddev/ddev-drupal-contrib), you may need to [change the symlink location](https://github.com/ddev/ddev-drupal-contrib?tab=readme-ov-file#changing-...).