docs(glue-alpha): fix inconsistent structures in README

[ozelalisen]

Reason for this change

There are inconsistencies in the README structure. Fixed them into unified structure.

Description of changes

• I made Spark Jobs as subheader as they provide examples and core part of L2 construct. I kept triggers as numbered list as they do not provide any examples and no need to highlight them.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

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