[Docs]: identity contains incorrect links

[lgecse]

Issue Description

Identified by #243, there are some invalid urls in the Identity docs:

WARNING -  htmlproofer: invalid url - https://google.github.io/A2A/specification/agent-card/ [404]
           [identity/arch_diagrams.md]
WARNING -  htmlproofer: invalid url - https://google.github.io/A2A/specification/agent-card/ [404]
           [identity/credentials.md]
WARNING -  htmlproofer: invalid url - https://google.github.io/A2A/#/documentation [404]
           [identity/identifier_examples.md]
WARNING -  htmlproofer: invalid url - https://google.github.io/A2A/specification/agent-card/ [404]
           [identity/identifier_examples.md]

Prposed Solution

Fix links to point to the actual content.

Affected Component

Other

Checklist

• I have read the contributing guidelines.
• I have verified this does not duplicate an existing issue.

[keraron]

Fixed in #283