[Section, Form]: Remove default level and titleVariant values, make title and level as required properties

[RiinaKuu]

https://funidata.atlassian.net/browse/DS-397

1. Section is part of the semantic order of the page. Initially Section title had a default aria-level value of 2 and variant of 'lg', but since we want to make sure that developers take into consideration the semantic order, following changes have been made:

• Set level as required property
• Removed level and titleVariant default values
• Adjust titleVariant logic to match the corresponding level which can be overriden

2. Same kind of changes in Form Component as well

• Set title and level as required properties
• Removed titleVariant default value and adjust its logic to match the corresponding level which can be overriden

3. Updated docs and created Typography util function which is used in Heading and Section components.

[videoeero]

LGTM