Create a password management guide

[a-velasco]

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). Subsection for replica sets and sharded clusters as needed