Add docs for creating a bespoke banner

We had to deploy a banner for some org pages on 2ndline as a result of a
request during a mini-reshuffle. This documentation should be make it
easier if we need to deploy another banner.

source/manual/deploy-banner-to-certain-pages.html.md

@@ -0,0 +1,51 @@
+---
+owner_slack: "#govuk-2ndline-tech"
+title: Deploy a banner to certain pages
+parent: "/manual.html"
+layout: manual_layout
+section: Frontend
+---
+
+This documentation should be used when we receive a request to publish a banner
+to a single, or group of pages. If you want to publish a sitewide banner then
+reference [deploy emergency banner][emergency-banner] or [deploy non-emergency
+banner][non-emergency-banner].
+
+We can publish a few different banner types, depending on the ask. For example,
+we've previously published a [notice banner][notice-banner] for a
+mini-reshuffle and an [intervention banner][intervention-banner] for a research
+panel.
+
+The [component guide][component-guide] has a list of GOV.UK components which
+includes banners like [notice][notice] and [intervention][intervention] and
+their variants, e.g [notice with markup in title][notice-markup].
+
+## Prerequisites
+
+* banner text
+
+* banner location on page/s
+
+## Steps
+
+1. Find the rendering app of the chosen pages (visit `https://gov.uk/api/content/<base-path>` and look for `rendering_app`)
+1. Create a branch introducing the component in the relevant view and with a helper method that only displays the banner for certain base_paths (e.g [org pages in Collections][notice-banner]).
+1. Preview the banner locally (e.g using docker [collections-app-live][collections-live], or deploy a branch to integration)
+1. Check the positioning of the banner and adjust accordingly. See [an example][notice-margin] of adjusting the margin of the notice banner.
+
+Once your changes are merged, you should also [write a technical debt
+card][tech-debt-board] for the banner you've added, as this is a non-standard
+piece of technical work that has been essentially 'hacked in' due to
+limitations in our current platform.
+
+[collections-live]: https://github.com/alphagov/govuk-docker/blob/main/projects/collections/docker-compose.yml#L41
+[component-guide]: https://components.publishing.service.gov.uk/component-guide/intervention
+[emergency-banner]: https://docs.publishing.service.gov.uk/manual/emergency-publishing.html
+[intervention]: https://components.publishing.service.gov.uk/component-guide/intervention
+[intervention-banner]: https://github.com/alphagov/collections/pull/3046
+[non-emergency-banner]: https://docs.publishing.service.gov.uk/manual/global-banner.html
+[notice]: https://components.publishing.service.gov.uk/component-guide/notice
+[notice-banner]: https://github.com/alphagov/collections/pull/3172/files
+[notice-margin]: https://github.com/alphagov/government-frontend/blob/8e8b10768c360c3c10b2f013086de2455e155040/app/views/content_items/corporate_information_page.html.erb#L8-L12
+[notice-markup]: https://components.publishing.service.gov.uk/component-guide/notice/with_markup_in_the_title
+[tech-debt-board]: https://trello.com/b/oPnw6v3r/govuk-tech-debt