[PENNYPALOOZA] Remove unused rst (#1559)

We no longer use the `demonstrations.rst` file or the CircleCI config
for this project. Sphinx still needs an `index.rst` file to build, but
it can be (and now is) empty. Removing these clears up a lot of the
warnings that Sphinx was previously throwing during builds.

**Note**: Also cleaned up a small config bug with Sphinx. Newer versions
require you to set the `language` parameter.

Example of building the gbs demo with these changes:
```
Running Sphinx v5.3.0
Using pandoc version: 3.7.0.1 to convert rst text blocks to markdown for .ipynb files
loading pickled environment... done
generating gallery...
generating gallery for demos... [100%] gbs.py                                                                                                    
building [mo]: targets for 0 po files that are out of date
building [json]: targets for 3 source files that are out of date
updating environment: 0 added, 3 changed, 0 removed
reading sources... [100%] demos/sg_execution_times                                                                                               
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] index                                                                                                                   
generating indices... genindex done
writing additional pages... search done
copying images... [100%] demos/images/thumb/sphx_glr_gbs_thumb.png                                                                               
copying downloadable files... [100%] demos/demos_jupyter.zip                                                                                     
copying static files... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
Sphinx-Gallery gallery_conf["plot_gallery"] was False, so no examples were executed.
build succeeded.

You can now process the JSON files in _build/json.
```

Author: doctorperceptron

.circleci/config.yml

@@ -35,7 +35,7 @@
 
 project = "PennyLane"
 
-copyright = "2022, Xanadu Quantum Technologies, Inc."
+copyright = "2025, Xanadu Quantum Technologies, Inc."
 
 author = "Xanadu Inc."
 
@@ -44,7 +44,6 @@
 # The full version, including alpha/beta/rc tags
 release = ""
 
-
 # -- General configuration ---------------------------------------------------
 
 # If your documentation needs a minimal Sphinx version, state it here.
@@ -148,7 +147,7 @@
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+language = "en"
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.