bpo-31487: Improve f-strings documentation (GH-3604)

[miss-islington]

Provide additional examples of using format specifiers in f-strings
Added examples for using integer and date format specifiers.
(cherry picked from commit f361897)

https://bugs.python.org/issue31487

[the-knights-who-say-ni]

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[miss-islington]

@Mariatta: Backport status check is done, and it's a failure ❌ .

[miss-islington]

@Mariatta: Backport status check is done, and it's a failure ❌ .

[miss-islington]

@Mariatta: Backport status check is done, and it's a failure ❌ .

[miss-islington]

@Mariatta: Backport status check is done, and it's a failure ❌ .

[miss-islington]

@Mariatta: Backport status check is done, and it's a failure ❌ .

[miss-islington]

@Mariatta: Backport status check is done, and it's a failure ❌ .

[miss-islington]

@Mariatta: Backport status check is done, and it's a failure ❌ .

[miss-islington]

Thanks, @Mariatta!