Update H2O-3 user guide to adhere to Makersaurus style guide

[hannah-tillman]

• [x] Index: updating user guide page to adhere to style guide - Hannah
• [ ] Welcome: updating user guide page to adhere to style guide - Hannah
• [x] API-related changes: updating user guide page to adhere to style guide - Hannah
• [ ] Cloud integration: updating user guide page to adhere to style guide - Hannah
• [x] Downloading & Installing: updating user guide page to adhere to style guide - Hannah
• [x] Starting H2O: updating user guide page to adhere to style guide - Hannah
• [x] #16212 - Hannah
• [x] #16213 - Hannah
• [x] #16214 - Hannah
• [x] #16215 - Hannah
  • [ ] #16216 - Shaun
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[hannah-tillman]

[ - name] means the ticket has been started [x] means the PR is merged