A glossary of terms for Parsl documentation

[Kanegraffiti]

This pull request addresses issue #3426 by adding a glossary of Parsl terms to the user guide and linking glossary terms throughout the first three sections of the User guide using citations.

Changes made:

• Created glossary.rst in the user guide section.
• Verified that the documentation builds successfully and all links work as expected by testing on the first 3 sections of the user guide.

Please review and let me know if further changes are needed.

[sophie-bui]

Hi @Kanegraffiti – please implement the following updates:

• Remove the numbers in front of terms. This will become tedious as we add more terms over time
• Review terms and organize them alphabetically according to the English alphabet (For example: "DataFuture" should go after "DataFlowKernel")
• Make sure the glossary directive is formatted according to these instructions to ensure we can link to the term throughout our docs

[WardLT]

Should I still review this or wait until after you address Sophie's comments?

[Kanegraffiti]

Should I still review this or wait until after you address Sophie's comments?

In this new PR, I have addressed @sophie-bui comments. Please review