docs: Cleanups for 1.18.0 release

[phlax]

Signed-off-by: Ryan Northey ryan@synca.io

Commit Message: docs: Cleanups for 1.18.0 release
Additional Description:

• picks fixes from [WIP] docs: Fix broken :refs: and other api issues #15766
• re-removes the admin section from the demo config and fixes docs
• fixes some more literal include lines
• prevent empty json dicts (see Docs are showing an empty dictionary in places #14995)

Risk Level:
Testing:
Docs Changes:
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue] Fix #14995
[Optional Deprecated:]
[Optional API Considerations:]

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to api/envoy/.
envoyproxy/api-shepherds assignee is @htuch
CC @envoyproxy/api-watchers: FYI only for changes made to api/envoy/.

🐱

Caused by: #15983 was opened by phlax.

see: more, trace.

[phlax]

re admin section, the reasoning for not including the admin section in the default config is that it exposes the server and i felt it was better to make adding the admin section explicit - so i added an admin section in getting started to compensate

i mistakenly approved #14833 that added this back when the better fix would have been to fix the docs

there is still the admin section in the actual docker image which we may want to follow up and remove

[phlax]

docs rendered here https://storage.googleapis.com/envoy-pr/15983/docs/index.html

[phlax]

@htuch ive added a fix to prevent the empty json dicts for extensions that dont have config

not sure if this is the correct fix - but its certainly better than what it does currently (at least where i have checked)

[phlax]

cc @mattklein123

[mattklein123]

Nice, thanks!