API reference: update tasks and keys

[maryamsulemani97]

Part of #1351 This PR updates the tasks and keys API reference to add an object heading and removes the tasks table from async operations page

For reviewers

• keys.md
  • We explain all the fields in the "Keys" object heading and then in the "Create key" endpoint. Is it a good idea to have this in two places?
  • I used an asterisk next to the required fields in the "Create key" endpoint. Is it visible?
• tasks.md
  • the tables in the "Details" section have different widths
• asynchronous_operations.md
  • Do we keep the summarized task object here?