docs: update network skeleton docs with devnet testing requirement

[BigLep]

This came from following these docs for nv27 in #13125

[Copilot]

Pull Request Overview

This PR updates the network skeleton documentation to include a new section on validating the network skeleton on a devnet, complementing the existing integration instructions.

• Renumbers steps for clarity.
• Provides instructions for devnet validation with updated references and examples.
• Updates text for post-PR submission instructions with links to example PRs.

Comments suppressed due to low confidence (2)

documentation/misc/Building_a_network_skeleton.md:189

• The step numbering now starts at 2, so please ensure that any preceding step is clearly defined to avoid confusion for readers.

2. Import new actors:

documentation/misc/Building_a_network_skeleton.md:254

• Please verify that the referenced URL for the local devnet documentation is current and accessible to maintain the accuracy and utility of the guide.

  - Have a local developer network that starts at the current network version.  See docs at https://docs.filecoin.io/networks/local-testnet .