[doc] add gva doc

doc/api/gva.md

+# GVA API
+
+GVA is a [graphql] API.
+
+## Schema
+
+See the playground of a GVA node:
+
+* https://g1.librelois.fr/gva
+* https://duniter-g1.p2p.legal/gva
+
+## Batch support
+
+It is possible to send a batch of graphql requests. The size of a batch is limited to 5 requests (No limit for whitelisted ip).
+
+## Anti-spam limitations
+
+These limitations apply only to non-whitelisted IPs:
+
+* The maximum number of requests per 20 seconds is 10.
+* The size of a batch is limited to 5 requests.
+* For paged requests, the pageSize parameter must be between 1 and 1000.
+
+[graphql]: https://graphql.org/