Fix dead links in documentation

[aram356]

Problem

Running npm run build in the docs/ directory fails with 16 dead link errors:

(!) Found dead link ./../crates/fastly/src/main.rs in file ONBOARDING.md
(!) Found dead link ./../crates/common/src/publisher.rs in file ONBOARDING.md
(!) Found dead link ./../crates/common/src/proxy.rs in file ONBOARDING.md
(!) Found dead link ./../crates/common/src/synthetic.rs in file ONBOARDING.md
(!) Found dead link ./../crates/common/src/integrations/registry.rs in file ONBOARDING.md
(!) Found dead link ./../trusted-server.toml in file ONBOARDING.md
(!) Found dead link ./../CONTRIBUTING in file ONBOARDING.md
(!) Found dead link ./../crates/common/src/integrations/testlight.rs in file ONBOARDING.md
(!) Found dead link ./../ProjectGovernance in file ONBOARDING.md
(!) Found dead link ./../README in file ONBOARDING.md
(!) Found dead link ./../AGENTS in file ONBOARDING.md
(!) Found dead link ./../SEQUENCE in file ONBOARDING.md
(!) Found dead link ./../FAQ_POC in file ONBOARDING.md

Additionally, cargo doc generates warnings for broken intra-doc links in crates/common/src/lib.rs referencing non-existent modules (gdpr, prebid, privacy, templates, why).

Root Cause

1. ONBOARDING.md: Links to source files (.rs) and root-level markdown files that VitePress cannot resolve
2. lib.rs: Module documentation lists modules that don't exist in the crate

Solution

1. Convert source file links to inline code format
2. Convert root markdown file links to full GitHub URLs
3. Update lib.rs module documentation to match actual public modules