chore: add documentation for password change feature

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Description

Documentation for password change feature. https://github.com/medic/cht-core/issues/9547

License

The software is provided under AGPL-3.0. Contributions to this project are accepted under the same license.

[latin-panda]

[latin-panda]

@esthermmoturi and @n-orlowski please review, not sure if this feature will make it for 4.15 release so I've put it in 4.16 for now

[latin-panda]

@n-orlowski I've applied the feedback, can you please review again?