docs: update README for mise installation and rustup command correction

[aryasaatvik]

Issue #, if available:

Description of changes:
discovered some issues while setting up

• Fix Rust command: rustup toolchain default stable → rustup default stable
• Update rtx → mise references (tool was renamed)
• Add missing mise installation steps

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 12.33%. Comparing base (f2e3632) to head (81cf42b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #353   +/-   ##
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  Coverage   12.33%   12.33%           
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  Files        2233     2233           
  Lines      190143   190143           
  Branches   174728   174728           
=======================================
+ Hits        23462    23463    +1     
+ Misses     165514   165512    -2     
- Partials     1167     1168    +1     

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