Add C# sdk to list of supported SDKs in overview doc

[edmundlam]

Update the SDK overview documentation to add the C# sdk to the list of supported SDKs

What type of PR is this?

/kind documentation

What this PR does / Why we need it:
Updates documentation to add C# to the list of a supported SDKs in the overview page

Which issue(s) this PR fixes:
n/a

Special notes for your reviewer:

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[edmundlam]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[agones-bot]

Build Succeeded 👏

Build Id: d8af632f-efe2-4d08-a984-708fe48e4d24

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.7.0-e1cfe8f
• image: gcr.io/agones-images/agones-sdk:1.7.0-e1cfe8f
• image: gcr.io/agones-images/agones-ping:1.7.0-e1cfe8f
• Linux C++ SDK (build): agonessdk-1.7.0-e1cfe8f-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.7.0-e1cfe8f.zip

A preview of the website (the last 30 builds are retained):

• https://e1cfe8f-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1678/head:pr_1678 && git checkout pr_1678
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.7.0-e1cfe8f

[markmandel]

Nice catch!

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: edmundlam, markmandel

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [markmandel]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment