(DOCSP-11251): Clarify that MongoDB Shell and mongosh are synonymous

[jeff-allen-mongo]

Staged: https://docs-mongodbcom-staging.corp.mongodb.com/mongodb-shell/docsworker-xlarge/DOCSP-11251/

[jeff-allen-mongo]

Hey @mmarcon,

I'm opening this PR to better clarify that MongoDB Shell and mongosh are synonymous in the sense the mongosh is the binary used for the new shell. For context, we're adding some content to the mongo shell docs that calls out the new shell and highlights the differences between the two. We don't want to be linking from a page that says mongo shell to a page that says MongoDB Shell because that seems like an easy way to confuse our readers. So, we're referring to the new shell as mongosh on the legacy mongo shell side and making these updates here to make it clear that mongosh == MongoDB shell.

Here is the content we're adding to the legacy shell docs if you want to take a look: mongodb/docs#4200

I hope this all makes sense. Feel free to ping or set up a call if you want to discuss this in greater detail.

And just to note, this is just a first pass. We'll likely need to adjust over time as we see how people react / interpret the callout, as well as when we start to transition users from the mongo shell to the new shell. If you have any suggestions for what to do for this pass let me know!