Branch out v0.49.x docs

[mstoykov]

What?

Releasing a fuele v0.49.0 version

Checklist

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• 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.

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