Update pytorch.org/ignite site and reference pytorch-ignite.ai #2325

[Moh-Yakoub]

Related to #2325

Description: I've updated some of the links as specified in the associated tasks and updated docs accordingly

To test it

1. cd docs
2. make html
3. cd build/
4. python -m http.server 8989

Checked the links and they referencing the updated links instead of the old html.

I didn't change the main URL (https://pytorch.org/ignite') to the new URL because it will break the pages not available for the new URL (for instance: examples), will change the base to the new URL once all of the URLs are available on the (pytorch-ignite.ai/) URL

Check list:

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

[vfdev-5]

@Moh-Yakoub thanks for the PR ! It looks good and we can merge it.
However, in order to finalize the migration we need to make a bit more updates to the API ref website. I'll update the issue description.
Basically, we need to have consistent reference to https://pytorch-ignite.ai
The index page have to minimally expose a reference https://pytorch-ignite.ai ...