PopART User Guide: Custom ops chapter review/fixes

Summary: This diff addresses the comments in the task and mainly adds links to the API docs.

Test Plan: !docs-only

Reviewers: #popart, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, shirazb, #tech_docs

Reviewed By: #popart, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, shirazb

Subscribers: shirazb

Maniphest Tasks: T55842

Differential Revision: https://phabricator.sourcevertex.net/D72354