Adding search shard routing docs

[AntonEliatra]

Description

Adding documentation for search shard routing

Issues Resolved

Part of https://github.com/opensearch-project/documentation-website/issues/4785

Version

all

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.

[hdhalter]

@rishabhmaurya , can you please do a tech review? Thank you.

[AntonEliatra]

@kolchfa-aws thank you for the review, I think I've now addressed all the points

[AntonEliatra]

@natebower @kolchfa-aws I've made the necessary changes, regarding the colon (question) - I'm not sure what the best way to proceed here is