docs: Use intersphinx to map old versions and cleanup version history (envoyproxy#16155)

This PR
- adds a patched version of intersphinx (~from sphinx-doc/sphinx#8981)
- uses versioned refs for version history
- cleans up inline literals in version history

Signed-off-by: Ryan Northey <[email protected]>
Signed-off-by: Gokul Nair <[email protected]>

Author: phlax

.yapfignore

@@ -5,3 +5,4 @@
 *_pb2.py
 *tests*
 *#*
+*intersphinx_custom.py