Change the environment variable that is used to suppress the nag message (#53)

The commit squidfunk/mkdocs-material@51d9b76 made this environment unusable - it's always set and there's no reasonable way to detect whether it was actually set on the command line.

Users will unfortunately run into a new nuisance even if they previously set our variable.

---

Also, add a direct mention how to suppress the warning.

Now that we've had to change this variable once, it's nice to give users a direct way to get rid of this.

Author: oprypin

properdocs/replacement_warning.py

@@ -31,6 +31,8 @@
 To avoid these risks, switch to *ProperDocs*, a continuation of MkDocs 1.x and a drop-in replacement that supports your current MkDocs setup.
 Simply install it with `pip install properdocs` and build your site with `properdocs build` instead of the MkDocs equivalents.
 
+Alternatively, to just skip this warning in the future, you can set the environment variable `DISABLE_MKDOCS_2_WARNING=true`.
+
 For more info visit https://github.com/ProperDocs/properdocs/discussions/33 and https://properdocs.org/
 
 (This warning was initiated by one of the plugins that you depend on.)
@@ -44,14 +46,10 @@ def setup():
         return
 
     if is_running_from_mkdocs():
-        # Allow to silence this warning with NO_MKDOCS_2_WARNING=true
-        if os.environ.get('NO_MKDOCS_2_WARNING', '').lower() != 'true':
+        # Allow to silence this warning with DISABLE_MKDOCS_2_WARNING=true
+        if os.environ.get('DISABLE_MKDOCS_2_WARNING', '').lower() != 'true':
             print(colorize_message(_warning_message), file=sys.stderr)  # noqa: T201
 
-    # Prevent warnings from the theme that already uses this environment variable.
-    # That warning does not apply to ProperDocs.
-    os.environ['NO_MKDOCS_2_WARNING'] = 'true'
-
     _warning_message = ''  # Disable all activations other than the first one.