feature request: document support multiple version

**Is your feature request related to a problem? Please describe.**
When user view the document https://www.apolloconfig.com/ , user may be only care about the document match their apollo's version.

For example, when user user's apollo version is 1.5.0, he/she may be confuse when see the feature above 1.5.0

**Describe the solution you'd like**
When someone use elasticsearch, its official site https://www.elastic.co/guide/en/elasticsearch/reference/7.17/index.html support choose the matched version of document.

Maybe we can add a button `version` on the top right of site?
and jump to that version's document which mark by the git tag.

**Additional context**
It's will improve the experience about the document, decrease the user's confuse.