Add detailed guidelines for contributors

[anovikov-el]

Description

Create comprehensive guidelines for contributors.
This will require changing CONTRIBUTING.md, SECURITY.md, adding the PULL_REQUEST_TEMPLATE subdirectory.

See the following links:
https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/about-community-profiles-for-public-repositories
https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository

Steps to reproduce

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Expected behaviour

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Actual behaviour

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Environment info

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Agree to the Code of Conduct

• I agree to follow this project's Code of Conduct