Add ONBOARDING, update TODO

ONBOARDING.md

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+# Onboarding
+
+A checklist of reminders for myself when installing this dotfile configuration.
+
+When **bolded**, this application won't be installed automatically. It implies that I need to install it manually.
+
+- Required applications
+  - [ ] age
+  - [ ] rbw
+  - [ ] bw
+- Semi-required applications
+  - [ ] **hishtory**
+  - [ ] **zsh**
+  - [ ] **oh-my-zsh**
+- Preferred applications
+  - [ ] kitty
+  - [ ] micro