docs: refactor lib/module document

[kazuk]

Issue number and link

Related Issue: #105

Describe your changes

crate / module documentation move to pure Markdown

crate documentation move to lib.md
module documentation move to [module_dir]/module.md

Checklist before requesting a review

• I follow the Semantic Pull Requests rules (bugfix/feature)
• I specified links to related issues (must: bugfix, want: feature)
• I have performed a self-review of my code (bugfix/feature)
• I have added thorough tests (bugfix/feature)
• I have edited ## [Unreleased] section in CHANGELOG.md following keep a changelog syntax (bugfix/feature)
• I {made/will make} a related pull request for documentation repo (feature)

[codecov]

Codecov Report

Merging #136 (b3a6ec5) into main (3001bbc) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #136      +/-   ##
==========================================
+ Coverage   87.80%   87.82%   +0.01%     
==========================================
  Files         203      203              
  Lines       12620    12620              
==========================================
+ Hits        11081    11083       +2     
+ Misses       1539     1537       -2     

Impacted Files	Coverage Δ
springql-core/src/expression.rs	75.44% <ø> (+1.19%)	⬆️
springql-core/src/lib.rs	100.00% <ø> (ø)
springql-core/src/pipeline.rs	92.30% <ø> (ø)
springql-core/src/stream_engine.rs	95.65% <ø> (ø)

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[laysakura]

I like to use include_str! but have some requests.

[laysakura]

sorry, I mistakenly made it ready (so made it back to draft again).

[laysakura]

👍

[laysakura]

[nits] I interpreted from "keep a changelog" and semantic versioning that Changed sections include "breaking changes", which bump up major versions.

But "keep a changelog" is a human readable format (not a machine readable one) and you noted Refactor:, it should be OK to add this line in CHANGELOG.