[docs] Install project in yaml instead of reqs (#1332)

Context: bd26f19

This (refactor, technically) was motivated by fixing a Dependabot error
that prevented it from creating new dependency PRs.
Error message:

> **Dependabot couldn't fetch all your path-based dependencies**
> The affected dependencies were `"-e ." at /requirements/docs.txt`.
>
> To use path-based dependencies with Dependabot the paths must be relative, resolve to a directory in this project's source code, and contain a valid Python project.

(From https://github.com/jazzband/django-simple-history/network/updates/827989501.)

Evidence that RTD still builds successfully:
https://readthedocs.org/projects/ddabble-django-simple-history/builds/24379182/

.readthedocs.yaml

@@ -21,3 +21,6 @@ sphinx:
 python:
   install:
     - requirements: requirements/docs.txt
+    # Install this project locally, so that its package metadata can be queried
+    - method: pip
+      path: .

requirements/docs.txt

@@ -1,4 +1,2 @@
-# Install this project in editable mode, so that its package metadata can be queried
--e .
 Sphinx==7.2.6
 sphinx-rtd-theme==1.3.0