Docs/update code sec docs and workflow

[sumau]

Description

• resolves Make Code Sec workflow clearer #51
• separates the security checklist from the security policy
• explains what this repo does in the README
• most of the changes are largely cosmetic apart from introducing a new requirement to complete training

Contributors

• @rowan-trade for creating the orginal version of the diagram

Type of change

• Refactoring (made code better without changing its behaviour)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation

How this has been tested

This is documentation so not testable, but I have made sure the links work correctly

Checklist

• I have performed a self-review of my code
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Reviewer Checklist

• I have reviewed the PR and ensured no secret values are present

[Freshrojek]

looks good to me, we can update as/when needed following ongoing discussions of when to bypass vulnerabilities

[github-actions]

Your PR has commits that are missing the Signed-off-by trailer. This is likely due to the pre-commit hook not being configured on your local machine. The usual fix for this issue is to run pre-commit install --install-hooks --overwrite -t commit-msg -t pre-commit, however for more detailed help in setting up the pre-commit hooks, follow the instructions at https://github.com/uktrade/github-standards/blob/main/README.md#usage

[github-actions]

Your PR has commits that are missing the Signed-off-by trailer. This is likely due to the pre-commit hook not being configured on your local machine. The usual fix for this issue is to run pre-commit install --install-hooks --overwrite -t commit-msg -t pre-commit, however for more detailed help in setting up the pre-commit hooks, follow the instructions at https://github.com/uktrade/github-standards/blob/main/README.md#usage

[github-actions]

Your PR has commits that are missing the Signed-off-by trailer. This is likely due to the pre-commit hook not being configured on your local machine. The usual fix for this issue is to run pre-commit install --install-hooks --overwrite -t commit-msg -t pre-commit, however for more detailed help in setting up the pre-commit hooks, follow the instructions at https://github.com/uktrade/github-standards/blob/main/README.md#usage

[github-actions]

Your PR has commits that are missing the Signed-off-by trailer. This is likely due to the pre-commit hook not being configured on your local machine. The usual fix for this issue is to run pre-commit install --install-hooks --overwrite -t commit-msg -t pre-commit, however for more detailed help in setting up the pre-commit hooks, follow the instructions at https://github.com/uktrade/github-standards/blob/main/README.md#usage