Small improvements for `CONTRIBUTING.md`

[chykon]

Description & Motivation

DEPENDS ON: https://github.com/intel/rohd/pull/327

It is proposed to make small changes to CONTRIBUTING.md:

• Add a link to SECURITY.md
• Place GitHub Discussions above GitHub Issues, as it's worth listing easier (for a random outsider) ways to get help first
• Fix names (vscode -> VSCode, StackOverflow -> Stack Overflow etc.)
• Add recommendation to use VSCode extension for Dart support
• Add a recommendation to use the recommended settings for the VSCode extension for Dart
• Update guidance on recommended file header template
• Update links
• Other small changes

Related Issue(s)

No.

Testing

Not required.

Backwards-compatibility

Is this a breaking change that will not be backwards-compatible? If yes, how so?

No.

Documentation

Does the change require any updates to documentation? If so, where? Are they included?

Yes, the guide for contributors is changing a bit.

[chykon]

Removed the ignore pattern that was added to https://github.com/intel/rohd/pull/327 (see https://github.com/intel/rohd/pull/327#issuecomment-1479056064)