Add ModelCheckpoint documentation example

[haochunchang]

Fixes #2622

Description:

• Add "See Example of Checkpoint" in the score_name of ModelCheckpoint.
• Change example from code block to actual test code (doctest).

Check list:

• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

Comments:

1. This is my first contribution to ignite, hopefully this PR can help newcomers to know how to use ModelCheckpoint more quickly.
2. I think the description of ModelCheckpoint handler can mention it is inherited from Checkpoint with convenient DiskSaver and last_checkpoint to use (if my understanding is correct).

[vfdev-5]

Thanks for the PR @haochunchang , lgtm !

[vfdev-5]

I think the description of ModelCheckpoint handler can mention it is inherited from Checkpoint with convenient DiskSaver and last_checkpoint to use (if my understanding is correct).

Yes, we should mention that if it helps to understand better the API. Thanks!
If you would like to update the PR with this suggestion, feel free

[haochunchang]

If you would like to update the PR with this suggestion, feel free

Thanks! I would like to update the PR to let us iterate on it.

[vfdev-5]

Lgtm, thanks @haochunchang