[DOCS] Link to `current release document` should point to current page in the current docs, when possible.

I consider some of this unexpected behavior a bug, but I've added some improvement suggestions that could be a feature.

**Description of the problem including expected versus actual behavior**:
Often I find docs via a Google search. The top result doesn't often land on the current version of Elastic. The banner that appears on top of the doc page suggests to go to the current release document. Unfortunately this often redirects the main index of the docs. Then I have to find the corresponding page via the index or go back to the original landing page and flip the version on the side drop-down or edit the url accordingly. I'd expect this link to send me to the same doc in the current version and if no longer valid go to the index of the page. Ideally, if it is no longer valid, it could include a deprecated message in the banner since going to the current docs would be misleading without at least a warning.

**Steps to reproduce**:
1. Go to a document page that isn't the current version, such as https://www.elastic.co/guide/en/elasticsearch/reference/6.x/geoip-processor.html
2. Observe the notification banner suggesting to go to the `current release document`.
3. Click `current release document` link.
4. Notice you get sent to https://www.elastic.co/guide/en/elasticsearch/reference/current/index.html

Workaround scenario:
1. Go to a document page that isn't the current version, such as https://www.elastic.co/guide/en/elasticsearch/reference/6.x/geoip-processor.html
2. Select `Elasticsearch Guide` drop-down and select `7.12 (current)`, or whatever version is current at the time you look at this.