docs: restore the markdown extensions pymdownx.extra bundled

Replacing pymdownx.extra with bare superfences silently dropped abbr,
def_list, footnotes, and pymdownx.betterem. The missing betterem was
already visibly wrong: the 'application/*' and 'text/*' literals in the
streamable_http docstring paired their asterisks into spurious emphasis.
A full-site rendering diff shows that fix is the only change the four
extensions cause.

Author: maxisbey

mkdocs.yml

@@ -126,9 +126,13 @@ extra_css:
 
 markdown_extensions:
   - tables
+  - abbr
   - admonition
   - attr_list
+  - def_list
+  - footnotes
   - md_in_html
+  - pymdownx.betterem
   - pymdownx.details
   - pymdownx.caret
   - pymdownx.critic