chore(docs): Decrement level of chapter 'Lines'

[PhilippFeO]

Based on Parser lines, the Lines plugin is deprecated and the code also signals this to the user.

The current version of the docs implies by structure, Lines is on the same level as Parser lines and especially Legacy Regexes, that this is not deprecated. Also, Lines doesn't mention it's deprecated.

I propose decrementing the level of Lines, such that it appears below Legacy Regexes, implying it's deprecation automatically.

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I ran into the issue, because I contemplated Lines equal to the rest of the possibilities (or I forgot the deprecation warning in Parser lines).

[bosd]

Good catch — the deprecation being hidden in Parser lines only is a real gap. Two nits before this can merge:

1. If you demote Lines to ####, its own subsections Handling Multiple Lines per Item (line 284) and Recipes & troubleshooting (line 303) also need to drop from #### → ##### so the hierarchy stays consistent.
2. Even with the demotion, a reader deep-linking to #lines won't see the parent context. Consider adding an explicit warning at the top of the section, e.g.:

   #### Lines
   
   > **Deprecated.** Use [`parser: lines`](#parser-lines) instead. This top-level `lines:` key is kept for backwards compatibility and will be removed in a future release.

   You could keep or drop the structural demotion — the callout is the load-bearing signal.

Also: please rebase onto master (currently BEHIND). Thanks for spotting this!

[PhilippFeO]

If you demote Lines to ####, its own subsections Handling Multiple Lines per Item (line 284) and Recipes & troubleshooting (line 303) also need to drop from #### → ##### so the hierarchy stays consistent.

Ups, should have done this in the first place. I glanced over it to fast. Thank you!

Even with the demotion, a reader deep-linking to #lines won't see the parent context.

True. I'll go with the explicit warning. 🤝