pyproject.toml: add docs to the parts to be excluded (MR 2312)

as reported in #2355 pmmbootstrap wheel installs files to /usr/lib/python3.12/site-packages/docs

pyproject.toml

@@ -26,7 +26,7 @@ Homepage = "https://www.postmarketos.org"
 version = {attr =  "pmb.__version__"}
 
 [tool.setuptools.packages.find]
-exclude = ["aports", "keys", "test", "test.pmb_test"]
+exclude = ["aports", "docs", "keys", "test", "test.pmb_test"]
 
 [tool.ruff]
 # E402: module import not on top of file, not possible for testcases