[Summarization] Adding summary and hands-on exercise

[ehumph]

This PR adds the missing summary and hands-on exercise to the code summarization chapter.

[github-actions[bot]]

No broken urls! :tada: Comment updated at 2023-04-27 with changes from 30654ecf17a32eb26933503746d2e182feac821e

[github-actions[bot]]

No spelling errors! :tada: Comment updated at 2023-04-27 with changes from 30654ecf17a32eb26933503746d2e182feac821e

[github-actions[bot]]

Re-rendered previews from the latest commit:

• See preview of Bookdown here
• See preview of Coursera/Leanpub version here
• Download the preview of .docx file

Updated at 2023-04-27 with changes from 30654ecf17a32eb26933503746d2e182feac821e