Replace https://www.zephyrproject.org/careers/ with GitJobs

[caniszczyk]

https://www.zephyrproject.org/careers/ can use some GitJobs love!

[tegioz]

At the moment, GitJobs synchronizes members and projects automatically from the corresponding foundation's landscape. This information is used, for example, to let employers indicate that they are Zephyr members, which can be needed to setup the filters for the embed view. But it looks like the Zephyr Project doesn't have a landscape yet. Maybe they'd be interested in setting one up? :)