Link to new post_X tags

[phil-opp]

I followed the posts step-by-step from the very beginning again and fixed various things along the way (#311, #317, #318, #319). The resulting code can be found in the post_1, post_2, etc. tags. Instead of branching off the master branch, these new tags are completely unrelated to any other branch (i.e. they form a completely separate history).

This makes our backporting process much easier. With the previous system, we needed to cherry-pick a fix to all affected tags (and resolve the merge conflicts). Now we can just cherry-pick the fix once and rebase all following tags on their predecessor (e.g. post_3 on post_2, post_4 on post_3, and so on). The new post_X names make it easy to automate this rebasing process (rebase post_N on post_{N-1}).

This PR also fixes #303.

I plan to delete the old tags (e.g. entering_longmode, set_up_rust, etc.) in the next few days after I verified that all tag links have been updated. I hope that there are no external links to these tags…