feat(docs) add doc for timeinterval operator

[githubxiaowen]

Description:
add documentation for timeinterval

Related issue (if exists):

[cartant]

The linting is failing on CI; some trailing whitespace needs to be removed:

ERROR: src/internal/operators/timeInterval.ts[26, 36]: trailing whitespace
ERROR: src/internal/operators/timeInterval.ts[27, 32]: trailing whitespace
ERROR: src/internal/operators/timeInterval.ts[33, 32]: trailing whitespace
ERROR: src/internal/operators/timeInterval.ts[34, 6]: trailing whitespace
ERROR: src/internal/operators/timeInterval.ts[39, 3]: trailing whitespace

[cartant]

Is see you've closed this, have made the requested changes and have opened another PR. That's fine, but you might find it easier - in the future - to make the changes in a new commit - on the same branch - and then to push that commit to your GitHub repo. That will see your PR updated with the new commit and the changes.

Anyway, thanks for your work. It's appreciated!