Add engineering career development framework

Created by: felixfbecker

This adds our career development framework proposal (the talent review process, and the level descriptions) to the handbook (see those documents for more context on our approach and the feedback that went into this). I added cross-links in the places I could think of. This was extensively reviewed in Google Docs so there should be no surprising content here :)

I experimented with a few different layouts and chose to go with a table to lay out the separate categories and visualize how they are axes all equally important for each level, but with bullet points within each cell, as not every bullet point "maps" to another bullet point in another category or level. I made the page a bit wider for this purpose, so that text doesn't wrap too much. There's a trade-off here in that this doesn't look good on mobile, but I think that's the right trade-off.

Other features:

• Individual levels can be linked to like headings on other pages.
• I used our brand colors for color-coding the categories and making the page visually appealing.
• Added sticky table headers so they are visible even when looking at higher levels.

The intro text is part of the levels page so the context on how to interpret them can't be missed. The second page about the talent review process is more for an EM audience (and hence there is some slight redundancy in content, but that's intentional).

We settled on the name "Career development framework" because the primary goal of this is to help ICs develop their careers and because it's, well, a framework to help with that.