Expand Elasticsearch architecture docs with examples on how to do certain changes

Conversation started in #328 (comment 250309518)

Basically we already have https://docs.gitlab.com/ee/development/elasticsearch.html#architecture which also talks about zero downtime migrations but doesn't give clear examples of how to accomplish it.

I believe we may have different strategies we can employ depending on exactly the kind of change you are making and documenting some best practices here will be really useful.

Some examples to consider:

1. Rename a field
2. Add a new field
3. Change an analyzer used during ingest
4. Change a search analyzer

I think some of these could in theory be done without a full migration using the multiple indices mappings and we could use some background migrations that happen during deployment. Depending on the volume of the changes need we may be able to make small migrations happen very quickly.

See some examples in of tools we have in Elasticsearch to help at #328 (comment 250848584)