docs: split language reference into more files

[lars-reimann]

Summary of Changes

The page about expressions was very long, making it overwhelming and confusing. It's now split it into multiple pages, so important terms already show up in the navigation.

To keep everything at the same level of navigation, I've also split the page about statements into several ones, and grouped all remaining pages in "Basic Structure" and "Advanced" sections.

[github-actions[bot]]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ MARKDOWN	markdown-link-check	26		0	1.43s
✅ REPOSITORY	git_diff	yes		no	0.05s

See detailed report in MegaLinter reports _Set VALIDATE_ALL_CODEBASE: true in mega-linter.yml to validate all sources, not only the diff_

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (3d43d38) to head (34be643).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1105 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 108 108 Lines 20816 20816 Branches 4091 4091 ========================================= Hits 20816 20816 ```

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[lars-reimann]

:tada: This PR is included in version 0.14.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• npm package (@latest dist-tag)
• v0.14.0
• GitHub release

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