docs: add THOR 10 to 11 upgrade guide

[Neo23x0]

Release hold: merge only when THOR 11 is released. Keep this pull request in draft until the release and engineering review are complete.

Adds an upgrade guide to the THOR 10 manual on master, linked from the contents, Update, and Configuration chapters. It explains how to obtain a fresh TechPreview package or upgrade an existing installation, and how to replace an old thor.yml with --reinitialize-config before reapplying custom settings.

The chapter covers renamed options, changed units and defaults, the new thor.yml.defaults reference, an estimated 40% increase in memory usage, and the --help short, --help long, and --help detailed options. In particular, it calls out the changed meaning of bare minmem values and the separate Info reporting threshold.

It also documents JSONL with JSON log format v3 as the new default, the explicit --text option, and possible differences in text logs reconstructed from THOR 11 objects. A separate section covers changed Thunderstorm response structures and the need to validate service clients before upgrading. The existing Output Options and Thunderstorm deployment sections link to this guidance.

Before merging at the THOR 11 release

• Incorporate engineering feedback and any additional migration requirements.
• Confirm the release upgrade/download commands.
• Confirm old-configuration warning behavior and handling of max_file_size_intense.
• Validate the memory estimate and documented defaults against the release build.
• Validate JSON/parser and Thunderstorm response migration guidance against the release build and add any further deployment-specific changes.

Validation

• Sphinx HTML build passes with warnings treated as errors.
• git diff --check passes.
• Generated HTML checks confirm the tables, cross-references, and hidden editorial notes.
• Both external reference links in the upgrade chapter pass the targeted link check.
• The full manual link check previously found an existing ASGARD helper-script 404 and timeouts for existing Splunkbase and VALHALLA links.

[phantinuss]

reimplemented in #220. This PR has the wrong merge base.