Make "events" in the document even clearer #1013

Description

Currently, we are using the term "event" for both Events API data and any incoming payload requests from Slack in the Bolt document. For example,

• There is the "Listening to events" section: https://slack.dev/bolt-js/concepts#event-listening which is referring to the Events API
• Then there is the "Acknowledging events" section: https://slack.dev/bolt-js/concepts#acknowledge which is referring to actions, shortcuts, commands, and options. Also, Bolt acknowledges the "events" for Events API under the hood.

This can be confusing for developers especially when they are not yet familiar with the Slack Platform yet.

To improve this, we can consistently use the term "request" for incoming requests from Slack. Specifically,

• We can rename the section "Acknowledging events" to "Acknowledging requests"
• We can replace all the phrases like "acknowledging events" to "acknowledge requests" in all sections

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• example code related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.