Add a section on "Sustainable travel guidelines"

Why is this change being made?

This change is based on the various university travel policies linked from the thread at https://twitter.com/JKSteinberger/status/1563135738312400897

Instead of a policy, I formulated these as guidelines as this is the first iteration, and GitLab might not want to force these guidelines at this stage.

The change is motivated by a few factors:

• the current climate situation
• virtual meetings are more climate-friendly than travel-based meetings
• air travel is proven to have the worst carbon footprint per kilometre per person

As an all-remote company, GitLab is positioned well to become a leader in the "climate-conscious" companies. We already save carbon emissions by not travelling regularly to the office and having most of our calls virtually.

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