Documentation: create a page explaining how to provide content to elements

[willoma]

A new page in the doc must exists, which explains:

• how elements read provided arguments (matching with types, etc)
• how and when to use With()
• how and when to use Withs()
• why there may be no content as a result, especially when providing a slice as an argument

[willoma]

Written in https://github.com/willoma/bulma-gomponents/commit/aad59990b2e5095bf71530643ad8d677cbbeff90