Added sub heading to ignite.metrics

[sayantan1410]

Related to #2437
Added a sub heading to ignite.metrics, however I have not added its link to other pages.
Let me know the changes that I need to make.

• Documentation is updated (if required)

[vfdev-5]

Thanks for the draft PR @sayantan1410
Let's improve text

[sayantan1410]

@vfdev-5 Hey, made the changes please check !!

[sayantan1410]

@vfdev-5 Hey I have made some changes, let me know if they are alright !!

[vfdev-5]

@sayantan1410 can you reread the docs https://deploy-preview-2448--pytorch-ignite-preview.netlify.app/metrics.html#ignite-metrics and critically estimate how more comprehensive or cumbersome docs are becoming now ?
For me there are few inconsistencies in the very beginning when you speak about accuracy (reader has directly the question why we talk about accuracy now). Code methods in text should be put in code-like ticks "`"

[sayantan1410]

@vfdev-5 Hey, I tried to make it simpler and consistent, can you please check it once more.

[vfdev-5]

Looks much better now, thanks @sayantan1410 !
Just a small nit

[vfdev-5]

@DhDeepLIT you were originally behind the request to improve our docs, can you please check if this PR provides enough info ? Thanks a lot !

https://deploy-preview-2448--pytorch-ignite-preview.netlify.app/metrics.html#attach-engine-api

[vfdev-5]

Thanks @sayantan1410
Let's wait a bit for other comments if any and merge it later.

[sayantan1410]

@vfdev-5 yeah, No problem !!