Fix instructions for docs dependencies and building

[Zaphoood]

Motivation and Context

The instructions in CONTRIBUTING.md for installing the dependencies for building the docs and the build command itself were wrong; this PR fixes this.

Regarding the installation command:

• the --dev flag is unnecessary since the dev dependency group is installed by default
• the --all-extras is unnecessary since the project doesn't use optional dependencies ('extras'). See docs here

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Public API Changes

• No Public API changes
• Yes, Public API changes (Details below)

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How Has This Been Tested?

I ran the two commands in question locally and verified that they both work.

Checklist

• The changes have been tested locally.
• Documentation has been updated (if the public API or usage changes).
• An entry has been added to CHANGELOG.md (if relevant for users).
• The code follows the project's style guidelines.
• I have considered the impact of these changes on the public API.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[Advueu963]

Thanks for pointing this out! I also do not see that we might need extras in the near future, but maybe @mmschlk has other plans