Move Datafusion Query Optimizer to library user guide

[devesh-2002]

Which issue does this PR close?

Closes #11497 .

Rationale for this change

Since, the optimizer Readme is not visible on the main docs page, this change was needed.

What changes are included in this PR?

I have tried to add the content of https://github.com/apache/datafusion/tree/main/datafusion/optimizer#readme in the Library User Guide in the docs.

Are these changes tested?

Yes, I have tested. However, I may need some more help.

[alamb]

Thanks @devesh-2002 ! This is looking like a great start

I built the docs locally and tested it out.

cd docs
./build.sh
open build/html/index.html

The new page doesn't seem to be in the index -- I think you have to add an entry here:

datafusion/docs/source/index.rst

Line 103 in b075ac4

library-user-guide/using-the-dataframe-api

Also, can you please remove the old copy at https://github.com/apache/datafusion/tree/main/datafusion/optimizer#readme and instead leave a link to the new location (e.g. I think the url will be

https://datafusion.apache.org/library-user-guide/<filename>.html

[alamb]

Perhaps can we name this file query-optimizer.md rather than datafusion-query-optimizer.md? I think the datafusion is redundant

[devesh-2002]

Thanks for the help! I have made some changes now. Please let me know, if any further changes are needed.

[alamb]

Thank you @devesh-2002 -- this looks great 🙏

I pushed a commit to resolve a CI failure and I think this looks good to god

[alamb]

I also filed #11581 to track updating this content

[alamb]

Thanks again @devesh-2002