Bug: 5762 Improve documentation for P2 transport cache and -U option

[Copilot]

Users are confused about how to force-refresh cached P2 repository data when repositories become available after a previous failure. The relationship between Maven's -U flag and Tycho's P2 transport cache was undocumented.

Changes

SystemProperties.md

• Added "Cache Behavior and the -U Option" section explaining cache semantics
• Documented that -U bypasses cache regardless of tycho.p2.transport.min-cache-minutes
• Updated min-cache-minutes property description to reference -U

Troubleshooting.md

• Added "Cached Repository Errors" section for the common "No repository found" scenario
• Documents why 404 responses get cached and how to resolve with -U
• Links to System Properties for detailed configuration

Example

When a repository was previously unavailable:

# Cache may contain stale 404 response - use -U to force refresh
mvn clean verify -U

Original prompt

TransportCacheConfig has a flag isUpdate. This effectivly maps to isUpdateSnapshots in the maven session and ultimativly to the -U option on maven CLI see https://maven.apache.org/ref/3.6.1/maven-embedder/cli.html
In #5762 it was brough up that this is currently badly documented so its not easy for users to understand the relation and even if we mentioned some of the properties used in the transport here https://tycho.eclipseprojects.io/doc/latest/SystemProperties.html the -U option is kind of magic to the users. Our task now is to review the tycho site doc and improve it at relevant locations to makes such things easier to discover for new users.

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