docs: add secret demo and doc

[ZLBer]

What this PR does:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Change the `secretStores` field in config.json to `secret_store`

[codecov]

Codecov Report

Merging #525 (2efacaf) into main (15daa68) will not change coverage.
The diff coverage is n/a.

❗ Current head 2efacaf differs from pull request most recent head 0d7a599. Consider uploading reports for the commit 0d7a599 to get more accurate results

@@           Coverage Diff           @@
##             main     #525   +/-   ##
=======================================
  Coverage   60.70%   60.70%           
=======================================
  Files         120      120           
  Lines        6382     6382           
=======================================
  Hits         3874     3874           
  Misses       2139     2139           
  Partials      369      369           

Impacted Files	Coverage Δ
pkg/runtime/config.go	60.00% <ø> (ø)

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[ZLBer]

@seeflood delete other demos

[seeflood]

Sorry I forgot to remind u that u should update the test-quickstart.sh. See the doc https://mosn.io/layotto/#/zh/development/test-quickstart?id=step-5-%e4%bf%ae%e6%94%b9-ci%e8%87%aa%e5%8a%a8%e6%b5%8b%e8%af%95%e6%96%b0%e5%86%99%e7%9a%84-quickstart-%e6%96%87%e6%a1%a3

[seeflood]

The quickstart failed:

It's caused by the k8s config in the json file and we can delete it

[ZLBer]

@seeflood done

[seeflood]

lgtm