Web IDE terminal documentation

What does this MR do?

This MR adds the documentation to configure and use the web IDE terminal feature

Related issues

Refs https://gitlab.com/gitlab-org/gitlab-ee/issues/5426

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23735

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23735)

/label ~Documentation