docs: add edge infrastructure concept page (Batch 3)

[marc0olo]

Summary

• Creates docs/concepts/edge-infrastructure.md covering API boundary nodes, HTTP gateways, the HTTP Gateway Protocol request lifecycle, and asset certification (certified variables for query responses)
• Adds edge infrastructure architecture diagram from Learn Hub (edge-infrastructure.png)
• Adds IC dashboard link for current API boundary node count
• Adds edge-infrastructure to the explicit Concepts sidebar after node-infrastructure
• Updates ic-http-gateway library link to renamed repo (dfinity/ic-http-gateway-protocol)
• Adds a "See also" link at the top of docs/references/http-gateway-spec.md
• The HTTPS Outcalls staging file (skipped article) is left untouched

Staging files deleted

• .migration/learn-hub/how-does-icp-work/icp-and-the-internet/icp-and-the-internet.md → docs/concepts/edge-infrastructure.md (intro section)
• .migration/learn-hub/how-does-icp-work/icp-and-the-internet/icp-edge-infrastructure.md → docs/concepts/edge-infrastructure.md
• .migration/learn-hub/how-does-icp-work/icp-and-the-internet/http-gateway-protocol.md → docs/concepts/edge-infrastructure.md#http-gateway-protocol
• .migration/learn-hub/how-does-icp-work/icp-and-the-internet/asset-certification.md → docs/concepts/edge-infrastructure.md#asset-certification

Sync recommendation

hand-written

[marc0olo]

Review notes

This PR is clean. All internal links resolve (http-gateway-spec.md, certified-variables.md, chain-key-cryptography.md), the bidirectional cross-reference with http-gateway-spec.md is correctly set, frontmatter is complete, and <!-- Upstream: --> comment is present. No banned patterns found.

Ready to merge.

[marc0olo]

Two issues to address before merging:

Em-dashes (banned project-wide)

docs/concepts/edge-infrastructure.md, lines 65–67 ("Further reading" bullets): three instances of — need to be replaced with : or parentheses.

docs/references/http-gateway-spec.md, "See also" line added by this PR: one instance of — needs the same treatment.

Per CLAUDE.md, em-dashes are banned in all prose, including bullet descriptions and "Further reading" / "See also" sections.

Missing section anchor

docs/concepts/edge-infrastructure.md, line 55: the link to chain-key-cryptography.md should be chain-key-cryptography.md#threshold-bls-signatures. The surrounding context is specifically about threshold BLS signatures, so a more precise destination is required per the project linking rules (always include a #anchor when the context refers to a specific subsection of the target page).

[marc0olo]

Feedback addressed:

• Replaced 3 em-dashes with colons in edge-infrastructure.md Further reading bullets (lines 65–67)
• Replaced em-dash with colon in http-gateway-spec.md See also line (line 8)
• Added #threshold-bls-signatures anchor to the chain-key-cryptography.md link at line 55 of edge-infrastructure.md (context is specifically about threshold BLS signatures)