WIP - [docs] Query type docs

[yesoreyeram]

This PR adds detailed documentation about various query types available in the plugin.

git clone https://github.com/grafana/github-datasource grafana-github-datasource
cd docs
make docs

visit http://localhost:3002/docs/grafana-github-datasource/latest/ and http://localhost:3002/docs/grafana-github-datasource/latest/query/ for details

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[MahmoudMabrok]

@yesoreyeram could u add docs for PullRequestreview