Add a :user: role in Sphinx

This change adds a role that links to the GitHub user Sponsors
page. If that page is not set up, it'll redirect to the GitHub user
profile page instead:

Links to https://github.com/sponsors/{{ username }} open as GitHub
Sponsors page if the target `username` has it enabled and redirect
to https://github.com/{{ username }} if it's disabled.

changelog.d/2395.doc.1.rst

@@ -0,0 +1 @@
+Added a ``:user:`` role to Sphinx config -- by :user:`webknjaz`

docs/conf.py

@@ -10,9 +10,18 @@
     cwd=os.path.join(os.path.dirname(__file__), os.path.pardir),
 )
 
+# -- Project information -----------------------------------------------------
+
+github_url = 'https://github.com'
+github_sponsors_url = f'{github_url}/sponsors'
+
 # -- General configuration --
 
-extensions = ['jaraco.packaging.sphinx', 'rst.linker']
+extensions = [
+    'sphinx.ext.extlinks',  # allows to create custom roles easily
+    'jaraco.packaging.sphinx',
+    'rst.linker',
+]
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
@@ -27,6 +36,11 @@
 # The name of the Pygments (syntax highlighting) style to use.
 pygments_style = 'sphinx'
 
+# -- Options for extlinks extension ---------------------------------------
+extlinks = {
+    'user': (f'{github_sponsors_url}/%s', '@'),  # noqa: WPS323
+}
+
 # -- Options for HTML output --
 
 # The theme to use for HTML and HTML Help pages.  Major themes that come with