Add xk6-webtransport extension to documentation

[kelseyaubrecht]

What?

New extension to enable the use of the webtransport protocol, using webtransport-go.

Checklist

Please fill in this template:

• I have used a meaningful title for the PR.
• I have described the changes I've made in the "What?" section above.
• I have performed a self-review of my changes.
• I have run the make docs command locally and verified that the changes look good.

Select one of these and delete the others:

If updating the documentation for the most recent release of k6:

• I have made my changes in the docs/sources/next folder of the documentation.
• I have reflected my changes in the docs/sources/v{most_recent_release} folder of the documentation.
• I have reflected my changes in the relevant (e.g. when correcting a documentation error) folders of the previous k6 versions of the documentation.

If updating the documentation for the next release of k6:

• I have made my changes in the docs/sources/next folder of the documentation.

Related PR(s)/Issue(s)

[CLAassistant]

All committers have signed the CLA.

[javaducky]

LGTM! Thank you!