* Update authentication concept doc.
* Fix lint errors.
* Address comments and fixed some links.
* Remove feature stages change from this PR.
I will make a separate PR for it.
* Consolidate the security concept pages into a single page.
- This updates the security concept material to be on a single page, which matches the
change done last week for the rest of the concept material. This ends up being a less clicky
more directed introduction for newcomers to the platform.
- While I was there, I moved the redundant What is Istio page from our about section and stuck
the content at the top of the What is Istio page in the Concepts section.
- Remove What's next sections since we now have auto-generated See also sections
- Fix a few incorrectly capitalized headers, "istio", "kubernetes", "sidecar"
mtail
Limin Wang
Axel Siebenborn
Tao Li
Martin Taillefer
Steven Dake
Yossi Mesika