[0.8.0] Add kernel troubleshooting docs

[eloquence]

As part of the 0.8.0 release, we are anticipating that we will ship an upgrade from grsecurity-kernel patched kernel version 4.4.115 to 4.4.135 (this will not impact instances with downgraded kernels). This is being done via #3494.

While we don't anticipate new issues with this kernel and will do hardware testing both on officially recommended hardware and on some additional server models, we must provide downgrade instructions to users in case of unanticipated issues that aren't immediately resolvable.

Implementation

For the 0.5 to 0.6 upgrade, we provided extensive kernel troubleshooting docs here:
https://docs.securedrop.org/en/release-0.7/upgrade/0.5.x_to_0.6.html

I suggest adding a generalized version of these instructions to the admin guide, and referencing them in the upgrade instructions for 0.7 to 0.8, instead of duplicating the whole page.