[docs] Improve rows documentation

[flaviendelangle]

• Remove strange wordings / repetitions like

Grid rows can be defined with the rows prop.
rows expects an array of objects.
Rows should have this type: GridRowModel[].

• Improve the updateRows demo to display all the basic cases (I could add a "delete" + "update" to show that we can put several type of actions into one update, but not sure it is worth it)

• New prop.rows update demo with the same features as the updateRows one

• Section order change to put the infinite loading section after the updateRows (to keep the two basic row updating sections next to eachother)

• New warning to encourage people to memoize their rows

• New link to the density demo to avoid having people updating the row height when they can do otherwise