fix: remove dashboard references from code tool docs

[vapi-tasker]

Description

• Remove dashboard/UI references from the code tool documentation page (fern/tools/code-tool.mdx)
• The code tool feature does not exist in the dashboard UI, so all instructions about creating code tools via the dashboard were misleading
• Replaced the dashboard-based "Creating a Code Tool" steps with API-focused instructions that link to the API reference
• Removed the "In the Dashboard" subsection under "Using Code Tools in Assistants"
• Adjusted the "Creating Code Tools via API" section wording (removed "also" which implied dashboard was the primary method)
• All API-related content, examples, best practices, and comparison table remain intact

Resolves: DEVREL-492

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work

[vapi-tasker]

CI Check Analysis

The preview-go check failure is a pre-existing issue unrelated to this PR's MDX documentation changes:

• The failure is in the Go SDK generation step (Preview SDKs workflow → preview-go job)
• The errors are Go compilation errors in auto-generated files (assistants.go, calls.go, types.go) from the Fern SDK generator
• The same Preview SDKs failure occurs on other recent branches (e.g., tasker/DEVREL-491-fix-docs-curly-brackets) and .github/workflows/release-all.yml also fails on main itself
• This PR only modifies fern/tools/code-tool.mdx (documentation content) — it cannot affect Go SDK code generation

Relevant checks:

• ✅ fern-check — PASSED (validates docs structure and API spec)
• ❌ preview-go — Pre-existing failure in Go SDK generation (not caused by this PR)

[github-actions]

🌿 Preview your docs: https://vapi-preview-d9363cb6-5ff0-4b7a-b17d-0bed09952bc5.docs.buildwithfern.com