fix: avoid panic in normalize link plugin

[sanyuan0704]

When the user uses relative path in link syntax, such as:

[cat](../../../assets/cat.png)

If the pointed path is out of root dir, we can print some warning to user instead of panic.

[github-actions]

Benchmark Results

Linux

group                main                                   pr
-----                ----                                   --
mdx_rs/example.md    1.02      2.0±0.02ms     2.8 MB/sec    1.00  1979.4±22.51µs     2.8 MB/sec

Windows

group                main                                   pr
-----                ----                                   --
mdx_rs/example.md    1.00      2.0±0.05ms     2.9 MB/sec    1.04      2.1±0.27ms     2.8 MB/sec