Backport PR pandas-dev#45370: DOC: add version dropdown to the online doc site

Author: jorisvandenbossche

doc/source/conf.py

@@ -225,11 +225,24 @@
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the
 # documentation.
+
+switcher_version = version
+if ".dev" in version:
+    switcher_version = "dev"
+elif "rc" in version:
+    switcher_version = version.split("rc")[0] + " (rc)"
+
 html_theme_options = {
     "external_links": [],
     "github_url": "https://github.com/pandas-dev/pandas",
     "twitter_url": "https://twitter.com/pandas_dev",
     "google_analytics_id": "UA-27880019-2",
+    "navbar_end": ["version-switcher", "navbar-icon-links"],
+    "switcher": {
+        "json_url": "https://pandas.pydata.org/versions.json",
+        "url_template": "https://pandas.pydata.org/{version}/",
+        "version_match": switcher_version,
+    },
 }
 
 # Add any paths that contain custom themes here, relative to this directory.

web/pandas/versions.json

@@ -0,0 +1,30 @@
+[
+    {
+        "name": "dev",
+        "version": "docs/dev"
+    },
+    {
+        "name": "1.4 (rc)",
+        "version": "pandas-docs/version/1.4"
+    },
+    {
+        "name": "1.3 (stable)",
+        "version": "docs"
+    },
+    {
+        "name": "1.3",
+        "version": "pandas-docs/version/1.3"
+    },
+    {
+        "name": "1.2",
+        "version": "pandas-docs/version/1.2"
+    },
+    {
+        "name": "1.1",
+        "version": "pandas-docs/version/1.1"
+    },
+    {
+        "name": "1.0",
+        "version": "pandas-docs/version/1.0"
+    }
+]