[TASK] Improve Install Tool UX for first-time users

Specifically mention FIRST_INSTALL file and recommend new users to read the Getting Started documentation at docs.typo3.org when warning that the Install Tool is locked.

Also adds an info box with a similar documentation recommendation to new users.

This ensures that new users always get directed to the documentation and that new users are told about the FIRST_INSTALL file, also if TYPO3 doesn't detect a new installation.

Also fixes some minor grammar.

Resolves: #102933
Releases: main, 12.4
Change-Id: I0efe032c9bfe4a619b80c6f620d53fe062c8abc3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82618


Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>