Two ways, either:

Route 1: A bot that gives the role after typing the command/keyword.

• Make a new role. Eg. Members
• Disable Read Messages permission for @everyone on all channels except #rules.
• Enable Read Messages permission for all channels for the Members role.
• For the #rules page, disable SEND MESSAGES for Members.
• (Optional) Hide the command somewhere in the rules so they're required to search for it.
• Setup the bot that will assign the role.

Route 2: A bot that auto-gives a role and removes it after being verified.

(Main perk for this is you'll be able to prune members for inactivity if you don't have self-assign roles.)

• Make a new role. Eg. Unverified
• Have a bot that auto assigns this role upon users joining the server.
• Disable Read Messages permission for Unverified on all channels except #rules.
• Enable Send Messages permission for Unverified on #rules.
• (Optional) Hide the command somewhere in the rules.
• Setup the bot that will remove the role.

Note: Putting the Send Message permission to NEUTRAL in the other channels will allow you to have a functioning MUTED role. (Muted role has Send Messages permission DENIED on ALL channels.)