Support generating ref docs for Firestore exp & lite (#3966)

* support ref doc generation for firestore exp

* update ref docs

Author: Feiyang1

.gitignore

@@ -88,6 +88,7 @@ packages-exp/**/temp
 
 # temp markdowns generated for individual SDKs
 packages-exp/**/docs
+packages/**/docs
 
 # files generated by api-extractor that should not be tracked
 tsdoc-metadata.json