* use kubectl consistently throughout for Istio API resource C.R.U.D operations
xref: https://github.com/istio/istio.github.io/issues/1843
* fix typo
* review comments
* remove unnecessary instructions to use `replace` instead of `create`
* fix linter in `zh` content
* 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.
- Add <github_file> <github_blob> and <github_tree> to make it simpler to link to the right
place on GitHub.
- Use these new sequences throughout the docs.
- Also, fix bad HTML generated for the TOC in certain cirsumstances.
- Fix extra blank line inserted at the bottom of indented code blocks.