Better structuring for documentation

[gobsponge]

Rename Miden base

• In the reader's perspective, I don't think "Miden base" makes much sense; esp. "base" does not have much relevance on it's own as a general term
• Instead, we may rename it as "Introduction"
• When the readers clicks "Miden base", what they see there is literally an introduction page of the Miden Rollup

[bobbinth]

Agreed, the new proposed structure is described here: 0xPolygonMiden/miden-docs#2