docs: Indicate latest Zulip version in installation and upgrade docs.

With copy-editing from tabbott, and also a migration to use
LATEST_RELEASE_VERSION, which will be correct even on the /latest/
paths.

Fixes #19695.

(cherry picked from commit 3b1cb0b25a)

docs/conf.py

@@ -20,7 +20,7 @@ from typing import Any, Dict, Optional
 # sys.path.insert(0, os.path.abspath('.'))
 
 sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), "..")))
-from version import ZULIP_VERSION
+from version import LATEST_RELEASE_VERSION, ZULIP_VERSION
 
 # -- General configuration ------------------------------------------------
 
@@ -36,6 +36,7 @@ extensions = [
 
 myst_enable_extensions = [
     "colon_fence",
+    "substitution",
 ]
 
 # Add any paths that contain templates here, relative to this directory.
@@ -105,6 +106,10 @@ pygments_style = "sphinx"
 # If true, `todo` and `todoList` produce output, else they produce nothing.
 todo_include_todos = False
 
+myst_substitutions = {
+    "LATEST_RELEASE_VERSION": LATEST_RELEASE_VERSION,
+}
+
 
 # -- Options for HTML output ----------------------------------------------