chore: updating files to make contributing easier and better

[nicholasgriffintn]

Resolves #320, Resolves #322

Description:

This PR updates the documentation for contributors and updates the CI process so that it is easier to contribute to this repo.

Changes:

• Updated Codeowners to include detl
• Added a new Code of Conduct document (https://github.com/bbc/simorgh/blob/latest/.github/CODE_OF_CONDUCT.md)
• Added a new Security document (https://github.com/bbc/simorgh/blob/latest/SECURITY.md)
• Changed references to the default branch from master to main
• Improved the PR template (https://github.com/bbc/simorgh/blob/latest/.github/PULL_REQUEST_TEMPLATE.md)
• Added a GitHub workflow to automate the testing of PRs
• Added a GitHub workflow to automate stale checks for issues and PRs

[github-actions]

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