Fix anchor links in JTBD lists

Why is this change being made?

The existing JTBD page has h4s, but you can't link to them. This makes it harder to specifically refer to a given part of the jobs list.

JTBDs already have slug values, which are suitable for use in anchor links. This change adds those slugs as id attributes to the headings, which makes anchors work.

Before: Clicking the ⛓ icon sends you to the top of the page, since it links to #. Link to live handbook.

Screenshot of where to click

After: Clicking the ⛓ properly links to the section you're in. Link to review app.

Screenshot

This also works for the standard JTBD list (separate from the Secure JTBD list), for example here in Plan.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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