Troubleshooting

[robertgraeff]

What this PR does / why we need it:

This PR extends the documentation:

• a page with general troubleshooting hints,
• a list of common issues.

These pages address a user who creates installations and blueprints, and who wants to understand why an installation fails. (These pages do not address an operator of a Landscaper).

Which issue(s) this PR fixes: Fixes #

Special notes for your reviewer:

Release note:

- Troubleshooting guide

[gardener-prow[bot]]

Skipping CI for Draft Pull Request. If you want CI signal for your change, please convert it to an actual PR. You can still manually trigger a test run with /test all