docs: fix l2 --> l1 message pricing

[ArielElp]

Description of the Changes

The current documentation does not correctly price l2 --> l1 messages. Messages are not discounted with the storage update discounts, and also have another associated event emission cost, which is now added to the docs.

PR Preview URL

https://starknet-io.github.io/starknet-docs/pr-1192/documentation/architecture_and_concepts/Network_Architecture/fee-mechanism/

Check List

• [x] Changes have been done against main branch, and PR does not conflict
• [x] PR title follows the convention: <docs/feat/fix/chore>(optional scope): <description>, e.g: fix: minor typos in code

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This change is 

[github-actions[bot]]

Your preview build is ready! ✨ Check the following link in 1-2 minutes: https://starknet-io.github.io/starknet-docs/pr-1192/documentation/ .

[github-actions[bot]]

Your preview build is ready! ✨ Check the following link in 1-2 minutes: https://starknet-io.github.io/starknet-docs/pr-1192/documentation/ .

[leo-starkware]

LGTM