ci: add spell checking with typos

[hendo-21]

Summary

References closed pyfenn/fenn issue #122.

This PR adds spell checking through typos (checking only, no corrections).

Changes

• Created .github/workflows/ci.yaml to run a GitHub action spell checking repo on PR
• Created _typos.toml to manage global spell-checking exclusions (e.g., ignoring theme HTML overrides, logo, CSS assets etc.).

Verification

I ran the pipeline locally inside a simulated Docker container via act to ensure the yaml syntax and toml exclusions are operational:

[Spell Check/run-spell-check] ⭐ Run Set up job
[Spell Check/run-spell-check]   ✅  Success - Set up job
[Spell Check/run-spell-check] ⭐ Run Pre Spell Check Repo
[Spell Check/run-spell-check]   ✅  Success - Pre Spell Check Repo
[Spell Check/run-spell-check] ⭐ Run Main Checkout Actions Repository
[Spell Check/run-spell-check]   ✅  Success - Main Checkout Actions Repository
[Spell Check/run-spell-check] ⭐ Run Main Spell Check Repo
[Spell Check/run-spell-check]         $ /tmp/typos .
[Spell Check/run-spell-check]   ✅  Success - Main Spell Check Repo
[Spell Check/run-spell-check] ⭐ Run Post Spell Check Repo
[Spell Check/run-spell-check]   ✅  Success - Post Spell Check Repo
[Spell Check/run-spell-check] ⭐ Run Complete job
[Spell Check/run-spell-check] Cleaning up container for job run-spell-check
[Spell Check/run-spell-check]   ✅  Success - Complete job
[Spell Check/run-spell-check] 🏁  Job succeeded

Notes

• typos found no spelling issues in the existing documentation

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