Add Tutorial for AttentionXML

[donglihe-hub]

What does this PR do?

Add tutorial for AttentionXML.

It seems that I have no rights to modify the bibliography.

Test CLI & API (bash tests/autotest.sh)

Test APIs used by main.py.

• [ ] Test Pass
  • (Copy and paste the last outputted line here.)
• [x] Not Applicable (i.e., the PR does not include API changes.)

Check API Document

If any new APIs are added, please check if the description of the APIs is added to API document.

• [ ] API document is updated (linear, nn)
• [x] Not Applicable (i.e., the PR does not include API changes.)

Test quickstart & API (bash tests/docs/test_changed_document.sh)

If any APIs in quickstarts or tutorials are modified, please run this test to check if the current examples can run correctly after the modified APIs are released.

[donglihe-hub]

@Eleven1Liu Please review the new documentation.

Sphinx uses global reference, which means all references will be one and only one place that lists all the references (Like a bibliography). That's why you can see AttentionXML in linear "handling many labels". I suggest we move the bibliography to another file.