[3.12] gh-85583: Add f-string index in tutorial/inputoutput.rst (GH-21681) (GH-126192)

gh-85583: Add f-string index in tutorial/inputoutput.rst  (GH-21681)

* bpo-41411 fstring index in tutorial/inputoutput

To assist in searching for fstrings I have added an index

* Add newline

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(cherry picked from commit a3443c0e22a8623afe4c0518433b28afbc3a6df6)

Co-authored-by: amaajemyfren <32741226+amaajemyfren@users.noreply.github.com>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Carol Willing <carolcode@willingconsulting.com>