ci(link-checker): use lychee instead of MLC

[Br1ght0ne]

This PR changes mdbook-docs.yml workflow to use https://github.com/lycheeverse/lychee ("Fast, async, stream-based link checker written in Rust") via https://github.com/lycheeverse/lychee-action instead of MLC (markdown-link-check).

The lychee config file (lychee.toml) closely mirrors the settings from existing mlc.mdbook.json, and thus should work with any and all Markdown files. I opted to leave mlc.next.json as-is for now, because it uses extra configuration.

Reasoning for the change:

• main reason - markdown-link-check operates on individual files (e.g., find . -name *md -exec markdown-link-check {};). This means that the errors are reported in the middle of the job output, which is quite long. Thus, it is difficult to find the actual error message. lychee uses globs like **/*.md natively (handled by lychee-action under the hood), meaning that there's a single report for all links in the repo's Markdown, with errors clearly visible. It looks like this by default (1 link failing on purpose):

  179/179 ━━━━━━━━━━━━━━━━━━━━ Finished extracting links                                                                                                                                                                                             Issues found in 1 input. Find details below.

[docs/src/connecting/external-node.md]:
✗ [403] https://faucet-beta-5.fuel.network/ | Failed: Network error: Forbidden

🔍 179 Total (in 1s) ✅ 177 OK 🚫 1 Error 💤 1 Excluded

• lychee can use GHA cache to avoid checking the same links on repeated CI runs - this is configured in this PR
• it's much faster than MLC, written in Rust and well-maintained (+ for security)