feat(process): document the release process with a mermaid diagram

docs/release-process.md

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+# Release process
+This diagram below represents the release process for MetaMask Extension and MetaMask Mobile clients.
+
+```mermaid
+graph TD
+
+%% Nodes outside subgraphs %%
+RUN[Runway] -->|every 2 weeks| CURRENT0
+CURRENT1 --> |create PR| bump1[GitHub Action creates version bump PR for the main branch]
+bump1 -->|merge PR| MAIN1
+CURRENT6 --> BUG1[A bug is found]
+BUG1 --> BUG2[A fix is done on 'main' branch]
+BUG2 --> CURRENT7
+FEAT[For every change, new features or fixes, engineers first open PRs on 'main'] -->|merge PR| MAIN1
+
+%% Subgraphs %%
+subgraph Main [Main developement branch: 'main']
+    style Main fill:#4d0808,stroke:#000,stroke-width:2px,color:#fff
+    MAIN1[All changes are first made on 'main' branch]
+end
+
+subgraph Previous [Previous release branch: 'release/x.y-1.z']
+    style Previous fill:#08084d,stroke:#000,stroke-width:2px,color:#fff
+    PREVIOUS1[Previous release is merged into 'stable' branch]
+    PREVIOUS1 --> PREVIOUS2['Stable Branch Sync' GitHun Action creates stable sync PR]
+    PREVIOUS2 -->|create PR| PREVIOUS3[Release engineer reviews and merges stable sync PR]
+    PREVIOUS3 -->|merge PR| MAIN1
+end
+
+subgraph HotFix [Hotfix release branch: 'release/x.y-1.z+1']
+    style HotFix fill:#08084d,stroke:#000,stroke-width:2px,color:#fff
+    HOTFIX1[Hot fix release is merged into 'stable' branch]
+    HOTFIX1 --> HOTFIX2['Stable Branch Sync' GitHun Action creates stable sync PR]
+    HOTFIX2 -->|create PR| HOTFIX3[Release engineer reviews and merges stable sync PR]
+    HOTFIX3 -->|merge PR| MAIN1
+end
+
+subgraph Current [Current release branch: 'release/x.y.z']
+    style Current fill:#08084d,stroke:#000,stroke-width:2px,color:#fff
+    CURRENT0[Runway automatically creates a new release branch based off of 'main' branch, called 'release/x.y.z']
+    CURRENT0 --> CURRENT1['Create Release Pull Request' GitHub Action is automatically executed]
+    CURRENT1 -->|create PR| changelog1[GitHub Action creates x.y.z changelog PR]
+    CURRENT1 --> CURRENT2[GitHub Action creates x.y.z release PR]
+    changelog1 -->|update PR| changelog2[Release Engineer reviews, adjusts, and merges x.y.z changelog PR]
+    changelog2 -->|merge PR| CURRENT4
+    CURRENT2 --> CURRENT5
+    CURRENT3[Release Engineer creates and merges stable sync PR into 'release/x.y.z branch'] -->|merge PR| CURRENT4
+    CURRENT4[A commit is added to 'release/x.y.z' branch] --> CURRENT5[A new release build is automatically created and posted on the x.y.z release PR]
+    CURRENT5 --> CURRENT6[Release is tested by all teams]
+    PREVIOUS1 --> CURRENT3
+    HOTFIX1 --> CURRENT3
+    CURRENT7[Release Engineer cherry-picks fixes on 'release/x.y.z' branch]
+    CURRENT6 --> CURRENT8[Release is approved by all teams]
+    CURRENT7 --> CURRENT8
+    CURRENT8 --> CURRENT9[Release is approved by Release Engineer, Release QA, and Release Manager]
+    CURRENT9 --> CURRENT10['Tag Release Branch' GitHub Action is triggered by Release Engineer]
+    CURRENT10 --> CURRENT11[Release tag is added on the release branch]
+    CURRENT11 --> CURRENT12[Release engineer merges x.y.z release PR into 'stable' branch]
+end
+
+subgraph Stable [Stable branch: 'stable']
+    style Stable fill:#26084d,stroke:#000,stroke-width:2px,color:#fff
+    CURRENT12 -->|merge PR| STABLE1[A new production build is automatically created and posted on the repo's releases page]
+    STABLE1 --> STABLE2[Release Engineer submits x.y.z production build to the store]
+end
+
+subgraph Next [Next release branch: 'release/x.y+1.z']
+    style Next fill:#08084d,stroke:#000,stroke-width:2px,color:#fff
+    STABLE2 --> NEXT1[Runway automatically creates a new release branch from main, called 'release/x.y+1.z']
+end