feat(walkthroughs): remove custom commands pro-tips and ollama walkthrough

[ichim-david]

• Removes the custom commands pro-tips from the edit and commands walkthroughs. This is because we no longer have documentation for it, we have instead the prompt documentation https://sourcegraph.com/docs/cody/capabilities/prompts#custom-commands
• Removes the ollama walkthrough. The walkthough was already removed in https://github.com/sourcegraph/cody/pull/7153

Test plan

• Open the Getting Started Guide
• Notice the ollama section missing
• We no longer mention the custom commands in the commands section