Add troubleshooting guide for update failures

[eloquence]

Update failures tend to be one of the most common issues during the SecureDrop Workstation pilot, with diskspace limits being one of the most common causes (cf. discussion in freedomofpress/securedrop-workstation#653 and upstream in QubesOS/qubes-issues#6676).

In the absence of other mitigations, we may want to add a section to the docs that explains some common steps to both detect & mitigate update failures:

• Interpretation of launcher.log and launcher-detail.log
• Analyzing diskspace usage
• Freeing up space using apt clean and apt autoremove
• Running updates manually in a template to debug other issues