DOCS Update SilverStripe Contributing Code GitHub labels

[sachajudd]

Suggested updates to SilverStripe CMS GitHub labels:
Note: I've created this pull request on behalf of @newleeland & @clarkepaul's work. cc @silverstripeux

Problem:
When reviewing the labels applied to both Type/UX and non-Type/UX issues they seem to be weighted quite differently, most noticeably when the phase “Workarounds” is used.

Example: When a CMS user has an alternative way to accomplish a task the label is often marked as Impact low (e.g. breadcrumbs don’t work as expected), because a user could choose to navigate the UI without using the breadcrumbs. The problem with this is that breadcrumbs might be the main way they intend to navigate and not be aware of alternatives. Breadcrumbs are a prominent feature of the UI, used in every view. When they don’t look or function correctly it is going to be very obvious to users that something is wrong, this can have a long term effect on how reliable they perceive our CMS to be and how confident they are with how we will manage their content and customer details.

• Enhancement label is not covered in the classifications
• Bug label is not covered (maybe this doesn't need to be?)

Proposed solution:

• Make it clearer that the labels are incorporating both tech functionality and end-user functionality.
• Add a definition of an enhancement
• Enhancements can be combined with the Impact rating and still make sense in that context
• Add definition of a bug

[dnsl48]

Makes sense to me. Impacts how we triage issues /cc @silverstripe/core-team

[maxime-rainville]

@silverstripe/core-team