Remove the configuration section in favor of concepts / reference (#13842)

Extends https://github.com/astral-sh/uv/pull/13841 — I'll drop that
commit later after that pull request merges but it's small.

I find the split into a "Configuration" section awkward and don't think
it's helping us. Everything moved into the "Concepts" section, except
the "Environment variables" page which definitely belongs in the
reference and the "Installer" page which is fairly niche and seems
better in the reference.

Before / After


<img
src="https://github.com/user-attachments/assets/80d8304b-17da-4900-a5f4-c3ccac96fcc5"
width="400">

.github/workflows/ci.yml

@@ -39,7 +39,7 @@ jobs:
 
           while IFS= read -r file; do
             # Generated markdown and JSON files are checked during test runs.
-            if [[ "${file}" =~ ^docs/ && ! "${file}" =~ ^docs/reference/(cli|settings).md && ! "${file}" =~ ^docs/configuration/environment.md ]]; then
+            if [[ "${file}" =~ ^docs/ && ! "${file}" =~ ^docs/reference/(cli|settings).md && ! "${file}" =~ ^docs/reference/environment.md ]]; then
               echo "Skipping ${file} (matches docs/ pattern)"
               continue
             fi