[Doribelove]

Fixes #235.

Add a troubleshooting page, linked from the documentation's main contents, for slow first plots caused by Matplotlib font-cache creation. Include cache-location diagnostics, a persistent MPLCONFIGDIR example, headless warm-up, and Docker/CI/sandbox guidance. Explain runtime-user permissions and why prewarming a different user's cache or mounting over it will not help.

Validation:

• Ran the documented Python commands in fresh Linux subprocesses with an initially empty cache (Python 3.12.14, Matplotlib 3.11.2). The warm-up creates a font-cache file; a second process reuses it without changing its contents or modification time. A plain import spatialmath does not populate the cache. Agg is confined to the warm-up command.
• The new page parses without Docutils warnings/errors. Checked the generated page, contents link, section anchor and the Matplotlib documentation link.
• Full Sphinx HTML builds of current upstream 41e0a4d and the revised documentation both complete with 21 warnings; existing diagnostic sets, including docutils/runblock errors, are identical. This does not claim a warning-free build.
• git diff --check passes.

Docker image builds and hosted-CI cache restoration were not executed. No cross-platform timing or performance guarantee is claimed.