Document how GOV.UK's sitemap is generated

This was a recommended action following a recent incident:
https://docs.google.com/document/d/1ycI4Zd-vIUl9xFz3InSLAOI_KS8uc_7Lg7Z5Xqt7OQ8/edit#heading=h.p3b9bm9twi19

source/manual/govuk-sitemap.html.md

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+---
+owner_slack: "#govuk-developers"
+title: GOV.UK's sitemap
+parent: "/manual.html"
+type: learn
+layout: manual_layout
+---
+
+GOV.UK's sitemap is available at https://www.gov.uk/sitemap.xml.
+GOV.UK is far too big to fit into one sitemap, so this file is more of a 'sitemap index', which references around 30 other XML files, such as `https://www.gov.uk/sitemaps/sitemap_1.xml`.
+
+## How the sitemap is generated
+
+Every morning, a [search-api-generate-sitemap cronjob](https://github.com/alphagov/govuk-helm-charts/blob/edc7c7ccd7972ff84f95e652e37570c1bd35e4e5/charts/app-config/values-production.yaml#L2254-L2256) runs to generate a fresh sitemap.
+
+The cronjob runs the [sitemap:generate_and_upload rake task](https://github.com/alphagov/search-api/blob/5636600a1dcb1517d30fe22334792b1e96537f6f/lib/tasks/sitemap.rake#L6) in search-api. This [enumerates over all documents in Search API](https://github.com/alphagov/search-api/blob/164b8ef982a7e360a05090676a158f05488365ce/lib/sitemap/generator.rb#L72) and [generates a sitemap](https://github.com/alphagov/search-api/blob/164b8ef982a7e360a05090676a158f05488365ce/lib/sitemap/generator.rb#L40-L41) matching the format specified in https://www.sitemaps.org/protocol.html. This job also [creates the sitemap index](https://github.com/alphagov/search-api/blob/164b8ef982a7e360a05090676a158f05488365ce/lib/sitemap/generator.rb#L32-L38).
+
+## How content gets into Search API
+
+The preferred pattern is for content to be published via Publishing API.
+After an edition is changed, Publishing API [publishes a message](https://github.com/alphagov/publishing-api/blob/6143731ffad5db48476d7647a75413c42a5224fd/app/services/downstream_service.rb#L40) to the `published_documents` topic exchange it [configured](https://github.com/alphagov/publishing-api/blob/52a763dc2bf29fd7038bdc6b8db3b617dfadafab/config/initializers/services.rb#L38-L49) on startup. Interested parties, such as Search API, can [subscribe to this exchange](https://github.com/alphagov/search-api/blob/a8045a2ef9d906e05d36a7708672e176dc4a3f8a/lib/tasks/message_queue.rake#L4-L13) to perform post-publishing actions.
+
+Search API [listens to the publishing queue](https://github.com/alphagov/search-api/blob/a8045a2ef9d906e05d36a7708672e176dc4a3f8a/lib/tasks/message_queue.rake#L15-L21) using the [govuk_message_queue_consumer gem](https://github.com/alphagov/govuk_message_queue_consumer). Its [MessageProcessor](https://github.com/alphagov/search-api/blob/ae8308de19a1521777ca1bd6a1a828efaef2c2d3/lib/indexer/message_processor.rb#L11) processes the indexing of the content.
+
+However, message queues aren't the only way to get content into Search API.
+Whitehall [calls Search API directly](https://github.com/alphagov/whitehall/blob/e748b577e0f13c01fe62bad2a303340ab5acc7c4/lib/whitehall/searchable.rb#L53), via [Whitehall::SearchIndex](https://github.com/alphagov/whitehall/blob/e748b577e0f13c01fe62bad2a303340ab5acc7c4/lib/whitehall/search_index.rb#L40), which is [called by](https://github.com/alphagov/whitehall/blob/a67fae1b8a0963927f38ce9987b99059fa9fff92/app/models/concerns/searchable.rb#L116) any model that includes the [Searchable](https://github.com/alphagov/whitehall/blob/a67fae1b8a0963927f38ce9987b99059fa9fff92/app/models/concerns/searchable.rb) module.