docs: fix serialization section links

[CattonNyan]

Summary

Fix two section links in the Payload Format section of the serialization guide.

Problem

The truncation link points to mavlink_2.md#packet_truncation, but that page has no matching anchor. The mavlink_parse_char() link points to use_libraries.md#receiving, which is a library overview rather than the C receiving guide.

Change

In en/guide/serialization.md:

Link truncation to the existing section on the same page (line 92):

- [truncates](../guide/mavlink_2.md#packet_truncation)
+ [truncates](#payload_truncation)

Link mavlink_parse_char() to the C library receiving guide (line 98):

- [mavlink_parse_char()](../getting_started/use_libraries.md#receiving)
+ [mavlink_parse_char()](../mavgen_c/index.md#receiving)

Verification

• Confirmed both destination sections in the Markdown source and their anchors on the published guide: payload truncation and C library receiving.
• Ran git diff --check and reviewed the diff; only the two link destinations change.
• Documentation-only change. No runtime testing, local documentation build, or lint scripts were run.