style: Rename subform component and add help text

[lassopicasso]

Description

This PR renames the subform component Underskjema to Tabell for underskjema and also adds help text to the component.
Thx for the help, @Ildest and @Annikenkbrathen 💪

Related Issue(s)

• itself

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.21%. Comparing base (2ffcc74) to head (2c51861).
Report is 25 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13995   +/-   ##
=======================================
  Coverage   95.21%   95.21%           
=======================================
  Files        1710     1710           
  Lines       22578    22578           
  Branches     2643     2643           
=======================================
  Hits        21498    21498           
  Misses        834      834           
  Partials      246      246           

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[JamalAlabdullah]

💯

[Jondyr]

Tested and looks correct✅