docs: demo contextValue in executeOperation (#7270)

Show an example of providing `contextValue` to `executeOperation`

<!--
First, 🌠 thank you 🌠 for taking the time to consider a contribution to
Apollo!

Here are some important details to follow:

* ⏰ Your time is important
To save your precious time, if the contribution you are making will take
more
than an hour, please make sure it has been discussed in an issue first.
          This is especially true for feature requests!
* 💡 Features
Feature requests can be created and discussed within a GitHub Issue. Be
sure to search for existing feature requests (and related issues!) prior
to
opening a new request. If an existing issue covers the need, please
upvote
that issue by using the 👍 emote, rather than opening a new issue.
* 🔌 Integrations
Apollo Server has many web-framework integrations including Express,
Koa,
Hapi and more. When adding a new feature, or fixing a bug, please take a
peak and see if other integrations are also affected. In most cases, the
fix can be applied to the other frameworks as well. Please note that,
since new web-frameworks have a high maintenance cost, pull-requests for
new web-frameworks should be discussed with a project maintainer first.
* 🕷 Bug fixes
These can be created and discussed in this repository. When fixing a
bug,
please _try_ to add a test which verifies the fix. If you cannot, you
should
still submit the PR but we may still ask you (and help you!) to create a
test.
* 📖 Contribution guidelines
Follow
https://github.com/apollographql/apollo-server/blob/main/CONTRIBUTING.md
when submitting a pull request. Make sure existing tests still pass, and
add
          tests for all new behavior.
* ✏️ Explain your pull request
Describe the big picture of your changes here to communicate to what
your
pull request is meant to accomplish. Provide 🔗 links 🔗 to associated
issues!

We hope you will find this to be a positive experience! Open source
contribution can be intimidating and we hope to alleviate that pain as
much as possible. Without following these guidelines, you may be missing
context that can help you succeed with your contribution, which is why
we encourage discussion first. Ultimately, there is no guarantee that we
will be able to merge your pull-request, but by following these
guidelines we can try to avoid disappointment.
-->

Co-authored-by: Trevor Scheer <trevor.scheer@gmail.com>

docs/source/api/apollo-server.mdx

@@ -581,6 +581,15 @@ The `executeOperation` method takes two arguments:
 - The first argument is an object describing the GraphQL operation to be executed.
   - Supported fields are listed in the table below.
 - The second argument is the optional options object. This object includes the operation's optional [`contextValue`](../data/context#the-contextvalue-object). This argument is only optional if your server _doesn't_ expect a context value (i.e., your server uses the default context because you didn't explicitly provide another one).
+
+```js
+const response = await server.executeOperation({
+  query: 'query SayHelloWorld($name: String) { hello(name: $name) }',
+  variables: { name: 'world' },
+}, {
+  contextValue: { userId: 'test' },
+});
+```
 
 The `response` object returned from `executeOperation` is a `GraphQLResponse`, which has `body` and `http` fields.