It is better from visibility PoV if the CONTRIBUTING.md file is visible at https://docs.victoriametrics.com/contributing/ .
This page can be indexed by search engines and searched by our users later.
It is also easier to provide a link to this page now comparing to the old link, which is much harder to remember:
https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/CONTRIBUTING.md
While at it, syncrhonize docs/CONTRIBUTING.md with .github/PULL_REQUEST_TEMPLATE/pull_request_template.md ,
so they do not contain duplicate information, which can be outdated over time. Now all the relevant information
is located at docs/CONTRIBUTING.md, while .github/PULL_REQUEST_TEMPLATE/pull_request_template.md refers this doc.
This is a follow-up for c006db1798
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/pull/6040
