docs: How Renovate Works overview - Basic (#25108)

Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>
Co-authored-by: Anne Stellingwerf <astellin@opentext.com>
Co-authored-by: Rhys Arkins <rhys@arkins.net>

docs/usage/key-concepts/.pages

@@ -1,4 +1,5 @@
 nav:
+  - 'How Renovate works': 'how-renovate-works.md'
   - 'Presets': 'presets.md'
   - 'Dependency Dashboard': 'dashboard.md'
   - 'Pull Requests': 'pull-requests.md'

docs/usage/key-concepts/how-renovate-works.md

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+---
+title: How Renovate works
+description: Learn how Renovate works
+---
+
+# Introduction
+
+Renovate usually performs these steps:
+
+- Cloning the repository
+- Scanning package files to extract dependencies
+- Looking up registries to check for updates
+- Applying any grouping rules defined
+- Pushing branches and raising Pull Requests
+
+Because Renovate must support a lot of dependency naming and versioning conventions, it has modules for each known convention.
+You can contribute your own modules, if you want.
+
+## Modules
+
+Renovate's modules are:
+
+- [datasource](../modules/datasource/index.md)
+- [manager](../modules/manager/index.md)
+- [platform](../modules/platform/index.md)
+- [versioning](../modules/versioning.md)
+
+Renovate uses these modules in order:
+
+1. The platform module interacts with the source control platform and clones the repository
+1. The manager module looks for files based on their name and extracts the dependencies (each dependency has a datasource)
+1. The datasource module looks up versions of the dependency
+1. The versioning module validates and sorts the returned versions
+
+For example:
+
+1. The `gitlabci` manager finds a dependency: `python:3.10-alpine` which has the `docker` datasource
+2. The `docker` datasource looks for versions and finds: `[python:3.9,python:3.9-alpine,python:3.10,python:3.10-alpine,python:3.11,python:3.11-alpine]`
+3. The `docker` versioning returns `python:3.11-alpine`, because that version is compatible with `python:3.10-alpine`
+
+# Workflow
+
+Here's an overview of the workflow:
+
+```mermaid
+flowchart TB
+    subgraph INITIALIZATION
+        direction TB
+        MC[Merge configurations \n most important to least: \n cli > env > file > default]
+        MC --> IP[Initialize platform]
+        IP --> AD[Query the platform for repositories]
+        AD --> NFIL[Narrow the list with filters]
+    end
+
+    subgraph REPOSITORY
+       direction TB
+       FER{{For each repository}}
+
+      subgraph EXTRACTD[EXTRACT DEPENDENCIES]
+          direction TB
+          CLBRANCH[Extract base branches]
+          CLBRANCH --> VULN[Check for vulnerabilities]
+          VULN --> CC{{For each manager}}
+          CC -->|manager A| CD["..."]
+          CC -->|manager B| CCF["match files"]
+          CCF --> CFEF{{For each file}}
+          CFEF -->|file 1| CCD1[Extract dependency]
+          CFEF -->|file 2| CCD2[...]
+      end
+
+      subgraph LOOKUP[LOOK UP UPDATES]
+          direction TB
+          UC{{For each manager}}
+          UC -->|manager A| UD["..."]
+          UC -->|manager B| UFEF{{For each file}}
+          UFEF -->|file 1| FED{{For each dependency}}
+          UFEF -->|file 2| FED2[...]
+          FED -->|dep 1| D1[...]
+          D1 -..-> CU
+          FED -->|dep 2| D2[use datasource to \n fetch versions]
+          D2 --> J[use versioning to find \n next valid update]
+          FED2 -...-> CU
+          UD -....-> CU
+          J --> CU[Look up updates]
+      end
+
+      subgraph WRITEU[WRITE UPDATES]
+        direction TB
+        FEU{{For each update}}
+        FEU --> AUCOND[Check if branch needed: \n existing/rebase/concurrent amount]
+        AUCOND --> AU[Create branch\nApply update\nCreate PR]
+      end
+
+      subgraph FINALIZE[FINALIZE]
+      direction TB
+        CM[Check for config migration]
+        CM --> CSB[Clean stale branches]
+
+      end
+
+      FER --> IRPO[Initialize repository]
+
+      IRPO --> EXTRACTD
+      EXTRACTD --> LOOKUP
+
+      LOOKUP --> WRITEU
+
+      WRITEU --> FINALIZE
+
+    end
+
+    INITIALIZATION --> REPOSITORY
+```