Split out the file and fqn selection methods into their own secti…

…ons (#4083)

### 🎩 Previews
- [The "file"
method](https://docs-getdbt-com-git-dbeatty-split-file-fqn-dbt-labs.vercel.app/reference/node-selection/methods#the-file-method)
- [The "fqn"
method](https://docs-getdbt-com-git-dbeatty-split-file-fqn-dbt-labs.vercel.app/reference/node-selection/methods#the-fqn-method)

## What are you changing in this pull request and why?

The `file` and `fqn` selections methods are distinct from each other,
but they've been documented within the same section. They deserve their
own sections, just like the rest of the methods.

### History

The `fqn` method was undocumented for some amount of time, and #1851
added the initial documentation.

### On con
- If anyone has a hyperlink that is using the previous anchor, it will
no longer go to that same section.

i.e., this will no longer exist:
-
https://docs.getdbt.com/reference/node-selection/methods#the-file-or-fqn-method

Instead, it will be split into these two:
-
https://docs.getdbt.com/reference/node-selection/methods#the-file-method
-
https://docs.getdbt.com/reference/node-selection/methods#the-fqn-method

But #1851 would have had the same issue (but in reverse), so in a sense,
we'd be just returning to the anchor that was in use previously.

Side note: there might be some way to override the value of the anchor
if we wanted it to be different than the default.

My recommendation is to just leave this PR as-is (without messing with
the auto-generated anchors).

## Checklist

- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
and [About
versioning](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#adding-a-new-version)
so my content adheres to these guidelines.

website/docs/reference/node-selection/methods.md

@@ -74,19 +74,28 @@ selectors unambiguous.
 
 <VersionBlock firstVersion="1.2">
 
-### The "file" or "fqn" method
-The `file` or `fqn` method can be used to select a model by its filename, including the file extension (`.sql`).
+### The "file" method
+The `file` method can be used to select a model by its filename, including the file extension (`.sql`).
 
 ```bash
 # These are equivalent
 dbt run --select file:some_model.sql
 dbt run --select some_model.sql
 dbt run --select some_model
-dbt run --select fqn:some_model # fqn is an abbreviation for "fully qualified name"
 ```
 
 </VersionBlock>
 
+### The "fqn" method
+
+The `fqn` method is used to select nodes based off their "fully qualified names" (FQN) within the dbt graph. The default output of [`dbt list`](/reference/commands/list) is a listing of FQN.
+
+```
+dbt run --select fqn:some_model
+dbt run --select fqn:your_project.some_model
+dbt run --select fqn:some_package.some_other_model
+```
+
 ### The "package" method
 
 The `package` method is used to select models defined within the root project