DEVDOCS-6413 Improve readability of modifier information in "Catalog …

[bc-Vince]

…- Product modifiers"

Added intro sections to provide detailed information on theoption_values array and the config object.

Added intro section with detailed information on supported file types for file upload modifiers.

Updated request and response bodies of endpoints to reference the new intro sections instead of having bulky descriptions.

DEVDOCS-6413

What changed?

• The introduction now has sections that go into detail about modifier option values, configs and supported file types.
• Relevant request and response fields now reference the new sections instead of including bulk information in their descriptions.

Release notes draft

• Added descriptive information on type-specific fields for modifier option values and configs.
• Added a table with information on supported file types for the file upload modifier type.

Anything else?

ping @bc-terra