Initialize Lean project

[eyihluyc]

This pull request initializes Lean4 formalization project. The focus of these initial commits is on establishing the project structure (which is still tentative) and introducing the minimal version of the calculus.
The following files were added:

• Minimal directory
  • Сalculus.lean specifies representation of 𝜑-calculus terms in Lean and contains implementation of reductions to both weak head normal form and normal form
  • Еest.lean contains conversion of terms to String and evaluates reductions of objects
• Extended directory, which is empty at the moment
• lakefile.lean contains information dependencies and how to build a project
• lake-manifest.json is auto-generated
• lean-toolchain specifies version of lean4
• PhiCalculus.lean is a build target

To-Do before merge:

• get feedback on the project structure for improvements

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PR-Codex overview

This PR focuses on adding the Minimal.Calculus module and implementing the Term and OptionalAttr types with their respective functions. It also includes a whnf function for weak head normal form evaluation and an nf function for normal form evaluation.

Detailed summary

• Added Minimal.Calculus module
• Implemented Attr, OptionalAttr, and Term types
• Implemented functions mapObj, incLocatorsFrom, incLocators, substituteLocator, lookup, insert, whnf, and nf
• Added ToString instances for OptionalAttr and Term
• Updated README.md with installation instructions

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[fizruk]

Slightly fixed package format and filenames, added CI workflow to build and generate docs, with caching. Once this PR is merged, documentation should be submitted to gh-pages branch, at which point we should enable Pages for this repository.