improve outposts docs

[krlosMata]

This pull request updates the outpost documentation to clarify how gas tokens are configured and represented on outpost chains. The changes provide detailed explanations of the parameters used for gas token configuration and distinguish between external tokens and native L2 tokens.

Gas token configuration and representation:

• Added descriptions of three key parameters for gas token configuration: gasTokenAddress, gasTokenNetwork, and gasTokenMetadata, including how metadata is encoded.
• Explained the distinction between external tokens (such as an outpost's native ETH representation) and native L2 tokens, with details on address derivation and metadata usage.

Documentation cleanup:

• Removed unnecessary line breaks at the end of the sequenceDiagram section to tidy up the documentation.