[docs] Improve fixed app bar placement section

[adeelibr]

This PR builds on improvements suggested in #17878

• I have followed (at least) the PR section of the contributing guide.

Closes #16844

[mui-pr-bot]

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[oliviertassinari]

Thanks for the follow-up pull request. In:

    <div>
      <Appbar position="fixed"></Appbar>
      <div className={classes.offset}> {/* to accomdate for top white space */}
    </div>

You can fix the syntax and apply our style convention.

[adeelibr]

I have made the changes, can you kindly review :)

[oliviertassinari]

Thank you for the updates. I have added a new commit, it was faster for me. Let me know what you think.

[adeelibr]

I think it is awesome now 👍👍👍👍