Add blurbs for individual charts

[CYX22222003]

What feature(s) would you like to see in RepoSense

Implementing the feature to support adding blurbs for each repo-author charts.

Is the feature request related to a problem?

If possible, describe the solution

We can follow the similar formats of Markdown file format in PR #2191. In addition, we may also need to create fields in the relevant summary type at the frontend to extract relevant markdown contents. Moreover, there is a need to change the layout of summary chart display in the frontend

If applicable, describe alternatives you've considered

Additional context