How to add/extend custom components in Gitlab UI

We have requirement to modify GitLab UI with custom components.
It is not an appearance or look and feel change, but adding our own UI attributes in GitLab UI.

For an example: consider the repository file in branch.
**We need to add notes to this file, these notes should display with our custom UI attribute. And also separately need to maintain these notes, without committing to the source file.

Note : These notes are separated from source fie commits.