docs: fix outdated piped config example in docs-dev contributing guide

[netram75]

What this PR does:
Updates the docs-dev contributing guide with two changes:

• Replaces the outdated platformProviders piped config example with the correct PipeCD v1 plugins syntax
• Adds "Contribute plugins" to the Ways to Contribute section

Why we need it:
The docs-dev contributing guide still used platformProviders — a v0 concept replaced by plugins in PipeCD v1. Any contributor following this guide to run piped locally would use the wrong config format.

Which issue(s) this PR fixes:

Refs #6679

Does this PR introduce a user-facing change?:

• How are users affected by this change: Contributors following the docs-dev guide now see the correct v1 plugin syntax for running piped locally
• Is this breaking change: No
• How to migrate (if breaking change): N/A