[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.

This issue covers migrating the following fields to Vue within a new "Advanced options" section:

URL masking - This field exists today but has no legend. Add the legend "URL masking"
Custom headers - Nested CRUD component (depends on !179072 (merged) 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)
Custom webhook template textarea
SSL verification checkbox
Handle any docs updates
- Eg. Go through https://docs.gitlab.com/api/get_started/get_started_extending/#step-2-set-up-webhooks carefully

Use the SettingsBlock component to contain the "Advanced settings" section
- Reference: 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)

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 (closed))

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) (merged) - Enable specifying a semantic element for CRUD components

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

Edited Dec 11, 2025 by Oiza Baiye