(DOCSP-39165) Node and RN: Change baseURL during runtime (#3243)

## Pull Request Info

Jira ticket: https://jira.mongodb.org/browse/DOCSP-39165

- [Node.js: Connect to an Atlas App Services
Backend](https://preview-mongodbkrollinsmdb.gatsbyjs.io/realm/DOCSP-39165/sdk/node/app-services/connect-to-app-services-backend/#connect-to-a-specific-server)
- [React Native: Connect to an Atlas App Services
App](https://preview-mongodbkrollinsmdb.gatsbyjs.io/realm/DOCSP-39165/sdk/react-native/app-services/connect-to-app-services-app/#connect-to-a-specific-server)

This PR adds content to both the Node.js and React Native SDKs for
changing the base URL during runtime. However, it only includes examples
for the Node.js SDK. Right now, the experimental module does not really
work for React Native. So, the React Native content notes that this
feature exists, but is experimental and points to the Node.js content.

Sorry for the large number of changed files. I couldn't resist removing
an old React Native app that we weren't using for anything.

Build errors:
- There's a build error about not being able to find
`label:node-change-server-during-runtime`. Manually testing shows the
ref link working.
- Looks like there are a couple flaky React Native tests. They're
unrelated to any changes I've made. I'll need to set aside time to look
at them later.

### Reminder Checklist

Before merging your PR, make sure to check a few things.

- [x] Did you tag pages appropriately?
  - genre
  - meta.keywords
  - meta.description
- [x] Describe your PR's changes in the Release Notes section
- [x] Create a Jira ticket for related docs-app-services work, if any

### Release Notes

- **Node.js SDK**
- App Services/Connect to an Atlas App Services Backend: Add new
sections and examples for updating the baseUrl, including during
runtime.
- **React Native SDK**
- App Services/Connect to an Atlas App Services App: Add new sections
that address updating the baseUrl.

### Review Guidelines


[REVIEWING.md](https://github.com/mongodb/docs-realm/blob/master/REVIEWING.md)

### Animal Wearing a Hat

<img
src="https://i.pinimg.com/1200x/59/fd/50/59fd501a656ce121c4c15ea52051c215.jpg"
width=400>

---------

Co-authored-by: lindseymoore <71525840+lindseymoore@users.noreply.github.com>

Author: krollins-mdb