docs: add validate warnings and refactor the account's functions page

[ArielElp]

Description of the Changes

Added warnings regarding important validations that should manually be added to __execute__. Slightly refactored the page, and removed most of the last section since the function's signatures don't add new information.

PR Preview URL

https://starknet-io.github.io/starknet-docs/pr-1302/architecture-and-concepts/accounts/account-functions/

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

---

This change is 

[starknet-bot]

Oops, your pull request failed to pass the Typo tests stage :( . see the result: error: absense should be absence --> ./components/Starknet/modules/architecture-and-concepts/pages/accounts/account-functions.adoc:19:112	19	At the moment of writing (Starknet 0.13.2), two critical validations must happen in +__execute__+, and their absense can lead to draining of the account's funds:	^^^^^^^

Please fix the typo, commit and push!

[starknet-bot]

Your preview build is ready! ✨ Check the following link in 1-2 minutes: https://starknet-io.github.io/starknet-docs/pr-1302/ .

[starknet-bot]

Your preview build is ready! ✨ Check the following link in 1-2 minutes: https://starknet-io.github.io/starknet-docs/pr-1302/ .

[starknet-bot]

Your preview build is ready! ✨ Check the following link in 1-2 minutes: https://starknet-io.github.io/starknet-docs/pr-1302/ .

[starknet-bot]

Your preview build is ready! ✨ Check the following link in 1-2 minutes: https://starknet-io.github.io/starknet-docs/pr-1302/ .

[starknet-bot]

Your preview build is ready! ✨ Check the following link in 1-2 minutes: https://starknet-io.github.io/starknet-docs/pr-1302/ .