[DOCS] structure improvements

[stufraser1]

To improve structure:

• ensure visual separation of user guide and context but keep them in the same documentation
• claims in the 4 bullets
• check and correct any naming inconsistency
• Move up the 'two kinds of catalogue' text - primary user guide, not decision-making consideration
• check and make clear 1 tool and the two packages
• consider removing repetition of Key Dec-making and fundamential guidance in step 10 / make more complementary