Create a password management guide

Create a simple how-to guide about how to manage passwords on MongoDB on replica sets and sharded clusters.

- [ ] Create file `docs/how-to/manage-passwords.md`
- [ ] Add to `docs/how-to/index.md`
- [ ] Create two general sections: how to retrieve a password, and how to set a password (see [MySQL example](https://canonical-charmed-mysql.readthedocs-hosted.com/how-to/manage-passwords/)). Subsection for replica sets and sharded clusters as needed

Start with the existing how-to structure in docs/how-to/index.md and the linked MySQL password-management example. Create docs/how-to/manage-passwords.md with sections for retrieving and setting passwords, covering replica sets and sharded clusters as needed, then add the guide to the index.

Written by the indexing model from the issue text.