Create a generic API query syntax document page

[newgene]

Relevant existing docs:

• MyGene.info:

  https://docs.mygene.info/en/latest/doc/query_service.html#query-syntax

• ES query string syntax:

  https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-query-string-query.html#query-string-syntax

We would like to create a generic API query syntax page at the BioThings SDK document (https://docs.biothings.io), so that each API instance can reference.