Rewrite WelcomePage to M3

[ArnyminerZ]

The PR should be in Draft state during development. As soon as it's finished, it should be marked as Ready for review and a reviewer should be chosen.

See also: Writing A Great Pull Request Description

Purpose

Update design of the welcome intro page so that it uses M3.

Short description

• Replaced M2's typographies with M3:
  • MaterialTheme.typography.substitle1 :: MaterialTheme.typography.bodyMedium
  • MaterialTheme.typography.h5 :: MaterialTheme.typography.labelLarge

Checklist

• [x] The PR has a proper title, description and label.
• [x] I have self-reviewed the PR.
• [x] I have added documentation to complex functions and functions that can be used by other modules.
• [x] I have added reasonable tests or consciously decided to not add tests.