Fix discovery external docs URL

[ziogaschr]

The RPC discovery document was returning links to GitHub repo of ethereum/go-ethereum. This PR makes those links, link back to our etclabscore/core-geth repo.

On top of that, this PR also adds link to our documentation site for the whole discovery document. This link has to be updated to the final page made available by #306.

Fixes #297

[meowsbits]

LGTM.

Example snippet provided as generated locally:

{
  "openrpc": "1.2.6", 
  "info": {
    "title": "Core-Geth RPC API", 
    "version": "1.11.22-unstable/generated-at:2021-01-20T15:07:56-06:00"
  }, 
  "externalDocs": {
    "description": "ETC Labs Documentation", 
    "url": "https://etclabscore.github.io/core-geth/"
  }, 
  "servers": [
    {
      "url": "ws://127.0.0.1:8546", 
      "name": "tcp"
    }
  ], 
  "methods": [
    {
      "name": "admin_addPeer", 
      "description": "```go\nfunc (api *privateAdminAPI) AddPeer(url string) (bool, error) {\n\tserver := api.node.Server()\n\tif server == nil {\n\t\treturn false, ErrNodeStopped\n\t}\n\tnode, err := enode.Parse(enode.ValidSchemes, url)\n\tif err != nil {\n\t\treturn false, fmt.Errorf(\"invalid enode: %v\", err)\n\t}\n\tserver.AddPeer(node)\n\treturn true, nil\n}// AddPeer requests connecting to a remote node, and also maintaining the new\n// connection at all times, even reconnecting if it is lost.\n\n```", 
      "summary": "AddPeer requests connecting to a remote node, and also maintaining the new\nconnection at all times, even reconnecting if it is lost.\n", 
      "paramStructure": "by-position", 
      "params": [
        {
          "name": "url", 
          "description": "string", 
          "summary": "", 
          "schema": {
            "type": ["string"]
          }, 
          "required": true, 
          "deprecated": false
        }
      ], 
      "result": {
        "name": "bool", 
        "description": "bool", 
        "summary": "", 
        "schema": {
          "type": ["boolean"]
        }, 
        "required": true, 
        "deprecated": false
      }, 
      "deprecated": false, 
      "externalDocs": {
        "description": "Github remote link", 
        "url": "https://github.com/etclabscore/core-geth/blob/master/node/api.go#L65"
      }
    },