Detailed icon functions and set usages

[adriendupuis]

Question	Answer
JIRA Ticket
Versions	5.0, 4.6
Edition

• Icon sets seems to be only for ibexa_icon_path() Twig function. (If there is a way to use icon set from config file, I didn't found how.)
• custom_icons.md was mostly focusing on content type icons while it's title suggest otherwise.
• icon_twig_functions.md should be exhaustive and also include ibexa_content_type_icon()
• icon_twig_functions.md heading levels were jumping from level 1 to level 3 without any level 2

(I get sucked in after #3026)

Checklist

• Text renders correctly
• Text has been checked with vale
• Description metadata is up to date
• Redirects cover removed/moved pages
• Code samples are working
• PHP code samples have been fixed with PHP CS fixer
• Added link to this PR in relevant JIRA ticket or code PR

[github-actions]

Preview of modified files

Preview of modified Markdown:

• docs/administration/back_office/back_office_elements/custom_icons.md
• docs/templating/twig_function_reference/icon_twig_functions.md

[mnocon]

Thank you!