Document the DateTimeImmutable support

[GautierDele]

Documents dateTimeImmutable(), added in xefi/faker-php#82 and released since, which was missing from the DateTime extension page.

Also documents that fromTimestamp / toTimestamp accept any DateTimeInterface — that PR widened them from DateTime to DateTimeInterface, so a DateTimeImmutable can now be passed as a bound too — as well as a unix timestamp int or a strtotime string. That was never spelled out on the page, which only ever showed the string form.

Checked each documented call against main:

dateTime            -> DateTime | 2023-06-04 23:26:41
dateTimeImmutable   -> DateTimeImmutable | 1998-12-21 05:45:19
timestamp(DTI, int) -> 1615724336 | 2021-03-14 | in range: true
dateTimeImmutable(DateTime, string) -> DateTimeImmutable | 2022-06-13

The other recently merged PRs need no doc change

• Update inconsistent return type faker-php#87 and Update inconsistent phpdoc annotations faker-php#88 — PHPDoc corrections on protected methods (addExtension, addLocaleExtension, packageManifestPath) and on ValidStrategy::pass(). Nothing changes in the public API, and the Strategies page already documents the valid() callable as returning a boolean, which is what #88 aligned the annotation to.
• 💄 Indent the mixin a better way faker-php#81 — indentation of the generated mixin. The Mixin generator section describes the mechanism but shows no sample output, so nothing to refresh.
• :rotating_lights: Fix unit tests does are NOK anymore faker-php#90 — CI matrix and a test assertion fix. No public surface.