docs: move plugin-architecture piped usage guide to official documentation

[A-d-i-t-y]

What this PR does

Moves the plugin-architecture Piped usage guide from
cmd/pipedv1/README-usage-alpha.md to the official documentation at
docs/content/en/docs-v1.0.x/installation/install-piped/plugin-architecture.md

Which issue this fixes

Fixes #6742

Changes

• Added new documentation page for plugin-architecture piped usage
• Replaced alpha notice with Hugo alert block
• Updated internal links to align with documentation pages
• Content includes: Prerequisites, Setup Control Plane, Run Piped, Deploy an Application

Does this PR introduce a user-facing change?

Yes - users can now find the plugin-architecture piped usage guide
in the official docs instead of having to look inside cmd/ directory.

[A-d-i-t-y]

Hi @Warashi @khanhtc1202 @eeshaanSA @ffjlabo @Ayushmore1214 @rahulshendre

This PR moves the plugin-architecture Piped usage guide from
cmd/pipedv1/README-usage-alpha.md to the official docs, improving
discoverability for users and contributors.

Would appreciate a review when you get a chance!

[A-d-i-t-y]

Hi @Warashi @khanhtc1202 @eeshaanSA @ffjlabo @Ayushmore1214 @rahulshendre

This PR moves the plugin-architecture Piped usage guide from
cmd/pipedv1/README-usage-alpha.md to the official docs.

Before: The guide was only accessible inside the cmd/ directory,
not visible in official docs.

After: Users can now find it directly in the installation section
of docs-v1.0.x.

Screenshots attached below:
Before -

After -

Would appreciate a review when you get a chance!