[Cloud Posture] Update plugin README with info on testing

[orouz]

Summary

re-visited the README file mostly to include info on FTR testing, did some cleaning too.
mostly left out info that isn't really specific for CSP

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 06fc56f

Metrics [docs]

Unknown metric groups

ESLint disabled in files

id	before	after	diff
osquery	1	2	+1

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	19	21	+2
fleet	61	67	+6
osquery	109	115	+6
securitySolution	439	445	+6
total			+20

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	20	22	+2
fleet	70	76	+6
osquery	110	117	+7
securitySolution	516	522	+6
total			+21

History

• 💚 Build #92158 succeeded 2ec6b28
• 💚 Build #92118 succeeded 2417c3e

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[elasticmachine]

Pinging @elastic/kibana-cloud-security-posture (Team:Cloud Security Posture)