docs: fix outdated links and various typos

[the-user-created]

This PR fixes several broken/outdated links and typos throughout the repository.

Link Updates

• Updated outdated links pointing to the original BlueBrain repository to their new location in the highfive-devs organization.
• Fixed a broken link to the github pages migration guide in CHANGELOG.md.
• Note: Links to specific historical items (e.g., discussions or issues) in the original repository are untouched to preserve context.

Typo Fixes

• Corrected a number of typos and grammatical errors in the documentation and github/CI config files.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[1uc]

Thank you!