Improve documentation of configuring external issue trackers

There was some confusion over the deprecated :issues_tracker_id field. Added documentation.

Closes #1514 (closed)

mentioned in issue #1438 (closed)

@marin, can you review this?

Added ~15078 ~20173 labels

Added 205 commits:

• 929d67c0...be05a9b9 - 204 commits from branch gitlab-org:master
• 81e066d9 - Improve documentation of configuring external issue trackers

@axil Thanks for the review! Updated.

Added 1 commit:

• c7e7306e - Revise documentation

Added 6 commits:

• c7e7306e...e22936c6 - 5 commits from branch gitlab-org:master
• 51294b9b - gi# This is a combination of 2 commits.

Added 7 commits:

• 51294b9b...16db51ec - 6 commits from branch gitlab-org:master
• 07dc5170 - Improve documentation of configuring external issue trackers

@marin, updated. Please review. Thanks!

Status changed to merged

mentioned in commit 59593158

Thanks