docs: Use modern sphinx-click extension syntax (#2615)

* Use 'sphinx_click' as the form of the Sphinx extension syntax, which
  has been adopted since sphinx-click v2.5.0.
* Set lower bound of v2.5.0 on sphinx-click requirement in 'docs' extra.

Author: matthewfeickert

docs/conf.py

@@ -58,7 +58,7 @@ def setup(app):
     'sphinx.ext.intersphinx',
     'sphinxcontrib.bibtex',
     'sphinx.ext.napoleon',
-    'sphinx_click.ext',
+    'sphinx_click',
     'nbsphinx',
     'sphinx_issues',
     'sphinx_copybutton',

pyproject.toml

@@ -102,7 +102,7 @@ docs = [
     "pyhf[xmlio,contrib]",
     "sphinx>=7.0.0",  # c.f. https://github.com/scikit-hep/pyhf/pull/2271
     "sphinxcontrib-bibtex>=2.1",
-    "sphinx-click",
+    "sphinx-click>=2.5.0",
     "pydata-sphinx-theme>=0.15.3",
     "nbsphinx!=0.8.8",  # c.f. https://github.com/spatialaudio/nbsphinx/issues/620
     "ipywidgets",