[Bug]: Missing SecurityRequirement section referenced in AgentSkill

### What happened?

The A2A specification documentation has broken links to `SecurityRequirement` in section 4.4.5 AgentSkill.

## Details

In the published documentation at https://a2a-protocol.org/latest/specification/#445-agentskill, the AgentSkill table references `SecurityRequirement` with a link to `#securityrequirement`, but this anchor does not exist on the page.

**Affected locations:**
- Section 4.4.5 AgentSkill - field `securityRequirements` links to `#securityrequirement`
- Also appears to affect the AgentCard section where `securityRequirements` field references the same

**Current state:**
- The `SecurityRequirement` message is defined in https://github.com/a2aproject/A2A/blob/7b900e77be2e4a9d8a3e505438b08a65a81b1ad2/specification/a2a.proto#L489
- Section 4.5 (Security Objects) contains sections 4.5.1 through 4.5.10
- None of these sections document `SecurityRequirement`

### Relevant log output

```shell
## Reproduction

1. Visit https://a2a-protocol.org/latest/specification/#445-agentskill
2. Click on the `SecurityRequirement` link in the `securityRequirements` field row
3. The link points to `#securityrequirement` which does not exist
```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

The issue is about a missing anchor link in the specification documentation. Start by locating the specification file (specification/a2a.proto) and the documentation generation process. Check how the SecurityRequirement message (line 489) is referenced in the generated docs. The fix likely involves updating the documentation source or the generation script to include the correct anchor. Verify by building the docs locally and checking the links.

Scritto dal modello di indicizzazione a partire dal testo della issue.