Draft: Add feedback border design tokens

What does this MR do?

• Adds feedback.*.border.color design tokens for:
  • strong
  • neutral
  • info
  • success
  • warning
  • danger
• Update examples to include border
• Update documentation for details on when a border is necessary

This is in support of #3162 and related issues where border colors are applied with constant values that could be replaced with semantics that aren't covered by action and control border design tokens.

Proposal

• Use border colors that are darker (light mode) or brighter (dark mode) than the default border so that feedback borders stand out.
• Use a more noticeable border for danger that matches the visual "loudness" of the feedback intent. For example, an error is visually louder than a success message. This is also for parity with control.border.color.error.

Visual with annotations…

Integrations

• GitLab: mr_url
• CustomersDot: mr_url
• Duo UI: mr_url
• Status Page: mr_url
• Docs: mr_url
• Switchboard: mr_url

Does this MR meet the acceptance criteria?

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Conformity

• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
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• Added the ~"component:*" label(s) if applicable.
• A changeset is added when this MR will do a patch, minor, or major update. More information can be found in doc/publishing-packages.md.

Components

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• If the MR changes a component's API, integration MR(s) have been opened (see integrations above).

Documentation

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Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
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Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.

Related to #3162