Generate readthedocs pages for the experimental examples group (mesa#3788)

`setup_examples_pages()` only scanned `basic/` and `advanced/`, so moving
alliance_formation into `mesa/examples/experimental/` dropped it from the
docs entirely. Drive the scan and the output-directory creation from a
single `kinds` tuple so the new group is picked up.

Also fix a typo in .gitignore: the generated overview page is
`docs/examples.md`, not `docs/example.md`, so it was showing up as
untracked after every doc build.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: codebreaker32

.gitignore

@@ -3,7 +3,7 @@ benchmarks/**/*.pickle
 
 # exampledocs
 docs/examples/*
-docs/example.md
+docs/examples.md
 
 # Byte-compiled / optimized / DLL files
 __pycache__/

docs/conf.py

@@ -323,27 +323,21 @@ def setup_examples_pages():
     # create md files for all examples
     # check what examples exist
     examples_folder = osp.abspath(osp.join(HERE, "..", "mesa", "examples"))
-    basic_examples = [
-        ("basic", f.path)
-        for f in os.scandir(osp.join(examples_folder, "basic"))
+    kinds = ("basic", "advanced", "experimental")
+    examples = [
+        (kind, f.path)
+        for kind in kinds
+        for f in os.scandir(osp.join(examples_folder, kind))
         if f.is_dir() and not f.name.startswith("__")
     ]
-    advanced_examples = [
-        ("advanced", f.path)
-        for f in os.scandir(osp.join(examples_folder, "advanced"))
-        if f.is_dir() and not f.name.startswith("__")
-    ]
-    examples = basic_examples + advanced_examples
 
     with open(os.path.join(HERE, "example_template.txt")) as fh:
         template = string.Template(fh.read())
 
     root_folder = pathlib.Path(os.path.join(HERE, "examples"))
     root_folder.mkdir(parents=True, exist_ok=True)
-    pathlib.Path(os.path.join(root_folder, "basic")).mkdir(parents=True, exist_ok=True)
-    pathlib.Path(os.path.join(root_folder, "advanced")).mkdir(
-        parents=True, exist_ok=True
-    )
+    for kind in kinds:
+        pathlib.Path(os.path.join(root_folder, kind)).mkdir(parents=True, exist_ok=True)
 
     examples_md = []
     for kind, example in examples: