Update Modular Tutorials post Redocly migration

[DennisDawson]

• Remove unused fields (private key, public key)

• Revise the Send Currency example and video per Issue #2092. (trust line is in the wrong direction)

• Make clear that amounts can be sent as XRP or tokens in most transactions (but not escrows).

• Update videos with revised UI captures

• Update screenshots

• Fix landing pages and navigation to highlight modular tutorials as the primary

• Add Checks tutorials