docs: fuel-101 transactions and bridge info

[sarahschwartz]

This PR adds 3 pages to the Fuel 101 section:

• Transaction & Block Models
• Transaction Lifecycle
• Understanding the Bridge

Please note that this PR doesn't include any graphics or visual aids, but these are planned on being added in the future. These docs are meant to be simple, high-level overviews that can be a starting point for more in-depth and accessible docs.