Configure self-hosted documentation in SPI manifest

[Kyle-Ye]

Summary

• Update .spi.yml to configure external documentation URL pointing to GitHub Pages hosted docs
• Remove mention of legacy SwiftPackageIndex documentation from README

This follows the Swift Package Index documentation for self-hosting documentation.

Test plan

• Verify .spi.yml is valid YAML
• Check that SPI recognizes the external documentation link

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 24.32%. Comparing base (d6cd066) to head (7d003df).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #542      +/-   ##
==========================================
+ Coverage   24.30%   24.32%   +0.01%     
==========================================
  Files         506      506              
  Lines       28979    28979              
==========================================
+ Hits         7044     7049       +5     
+ Misses      21935    21930       -5     

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