add initial docs for github pages

[liulanzheng]

What this PR does / why we need it:

add an initial doc in docs, which only has few content. any new documentations or updates are welcome.
This doc can be showed in github pages. Now only README.md is displayed, any markdown under /docs directory can be displayer if necessary in the futuer.

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?