documentation for k6 v0.48

[olegbespalov]

What?

This adds the documentation for the k6 v0.48 (cp -r docs/sources/next docs/sources/v0.48.x)

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/v{most_recent_release} folder of the documentation.
• I have reflected my changes in the docs/sources/next 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.

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