[Webhooks] Migrate Advanced options section to Vue, update Docs
## Summary
Complete the webhooks settings form migration to Vue by converting the "Advanced options" section from HAML to Vue components.
## Scope
This issue covers migrating the following fields to Vue within a new "Advanced options" section:
1. **URL masking** - This field exists today but has no legend. Add the legend "URL masking"
2. **Custom headers** - Nested CRUD component (depends on !179072 which is merged)
- Specify that the container tag should be `div` for this nested CRUD component to avoid nesting a semantic `<section>` within a `<section>` (this also resolves #513962)
3. **Custom webhook template textarea**
4. **SSL verification checkbox**
5. **Handle any docs updates**
* Eg. Go through https://docs.gitlab.com/api/get_started/get_started_extending/#step-2-set-up-webhooks carefully
## Implementation Details
- Use the `SettingsBlock` component to contain the "Advanced settings" section
- Reference: [http://localhost:9002/?path=/story/vue-shared-settings-settings-block--default](http://localhost:9002/?path=/story/vue-shared-settings-settings-block--default)
- Maintain the field order as specified above
- Ensure proper semantic HTML structure (no nested sections)
## Out of Scope
- Layout of existing hooks inside the CRUD component when not editing or adding
- Recent events section at the bottom of the page when editing an existing webhook
- Breadcrumbs and page titles consolidation (covered separately)
- Trigger checkboxes migration (covered in #578783)
## Related Issues
- Parent epic: #513631 - Convert Webhooks settings page to Vue and rearrange
- Related: #513962 - Section shouldn't be nested within a section
- Depends on: !179072 (merged) - Enable specifying a semantic element for CRUD components
## Acceptance Criteria
- [ ] Advanced options section is rendered using SettingsBlock component
- [ ] All four fields (URL masking, Custom headers, Custom webhook template, SSL verification) are migrated to Vue
- [ ] URL masking field has proper legend
- [ ] Custom headers CRUD component uses `div` container tag
- [ ] Tests are updated to reflect the new Vue structure
- [ ] No nested `<section>` elements in the DOM
- [ ] Docs are up to date
issue