Docs: use Docsy alert shortcode for notes and warnings

[aravindtga]

Description

Migrate all notes and warnings in the kpt documentation to use the Docs {{% alert %}} shortcode, aligning with the porch documentation style.

This change:

• Converts 5 usages of the custom {{< warning >}} shortcode to {{% alert %}}
• Converts 6 plain-text Note:/**Warning**: patterns to {{% alert %}}
• Deletes the custom documentation/layouts/shortcodes/warning.html (replaced by Docsy built-in)
• Fixes $primary SCSS variable from #fff to $kpt-dark-blue so color="primary" alerts render as visible blue (matching porch)
• Hardcodes navbar background to #fff in _nav.scss (matching porch) so changing $primary doesn't affect the navbar

Motivation

Addresses #4533

The kpt documentation had inconsistent note/warning formatting:

• A custom warning.html shortcode with non-standard syntax
• Plain-text bold **Note:** and **Warning:** without visual styling
• The $primary CSS variable was set to white, making color="primary" alerts invisible

After this change, kpt uses the identical alert pattern as porch:

{{% alert title="Note" color="primary" %}}
{{% alert title="Warning" color="warning" %}}
{{% alert title="Critical" color="danger" %}}

AI usage

I have used AI in the creation of this PR.

I have used the following AI tools:

• Kiro (Claude) to identify unconverted patterns, compare with porch styling, and generate the changes

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