Add cache plugin and tiered cache documentation

[sgup432]

Description

Adding a new doc for cache plugins introduced as part of 2.14. Also adding a doc for tiered cache which is one of the implementation provided via cache plugin.

Note: This is an experimental feature.

Issues Resolved

Closes https://github.com/opensearch-project/documentation-website/issues/6300

Checklist

• [x] By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and subject to the Developers Certificate of Origin. For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[sgup432]

@hdhalter This is almost the same as the previous one which got reverted. I have changed 2.13 to 2.14, hope it should be okay? There are few items/features missing but we will take those separately as the changes are not yet fully merged in.

[hdhalter]

Thanks, @sgup432 !

[sgup432]

I messed up a bit(other commits are added to this PR) trying to fix DCO check. Fixing it now.

[sgup432]

@natebower I have addressed your comments. Looks good to me.