Read local/relative markdown files support

Now `llms_txt2ctx` only supports to read `http://` or `https://` links, any idea to read relative markdown files? When editing `llms.txt`, relative files are friendly for IDE/Editor. It's is also friendly to convert local `llms.txt` to `llms.md`, and attach `llms.md` to chat bot.

```
# zawk

> zawk is an implementation of the AWK language written by Rust, and it includes a standard library for text, math,
> datetime, crypto, parser, encode/decode, KV, SQLite/MySQL, Redis/NATS, etc.

## Docs

- [FAQ](./info/faq.md): FAQ about zawk
```

Start by tracing how llms_txt2ctx currently handles http:// and https:// links, then check how a relative link such as ./info/faq.md in llms.txt is processed. Use the zawk example to verify local markdown files can be read and included when converting llms.txt to llms.md.

Written by the indexing model from the issue text.