[Automation] Modify workflow to, instead of PR, add comment with ReStructured Text

[PLeVasseur]

• Create new action to leave comment instead on issue, e.g. auto-conversion-as-comment.yml
• Update CONTRIBUTING.md to reflect process of PRs being opened
• Disable Coding Guidelines issue to PRs (auto-pr-on-issue.yml)

[PLeVasseur]

• Create new action to leave comment instead on issue, e.g. auto-conversion-as-comment.yml

Was accomplished by:

• feat: add ability to generate comment containing reStructured Text from Markdown #247
• fix: make all Python scripts able to see common scripts #249
• fix: rename file guideline_utils.py correctly #250
• http://github.com/rustfoundation/safety-critical-rust-coding-guidelines/pull/253

[PLeVasseur]

• Update CONTRIBUTING.md to reflect process of PRs being opened

Is waiting for this to merge:

• feat: update contribution workflow based on guideline generated as comment #251

Want others to make sure this reads cleanly.