docs: add doc for encryption password entry

[kevinssgh]

Description

Added brief description of the new password/encryption feature used by Zetaclient.

Closes: #1701

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Include instructions and any relevant details so others can reproduce.

• Tested CCTX in localnet
• Tested in development environment
• Go unit tests
• Go integration tests
• Tested via GitHub Actions

Checklist:

• I have added unit tests that prove my fix feature works

[lumtis]

I would rename the file

docs/zetaclient/migration_v12.2->v12.3.md

to keep it more generic in case we have other migration docs in the future