docs: documentation overhaul — theme, content, and terminology updates

[ruck314]

Summary

• Theme: Switch Sphinx theme from Furo to sphinx_rtd_theme to match the rogue documentation site; adds titles_only and navigation_depth options. Eliminates Furo-specific TOC errors in reference pages as a side-effect.
• Metadata: Update copyright year to 2026; replace jsmith placeholder username with smith across all .rst source files.
• Build pipeline: Add make gui interactive build section explaining use cases (exploration, synth review, XSIM, timing debug); rename "Partial Flows" section to "Dynamic Function eXchange (DFX) and Partial Build Flows" per AMD UG909 terminology.
• Recursion diagram: Rework the ::DIR_PATH two-level recursion example with explicit [SAVE]/[SET]/[RESTORE] annotations and a "What to notice" callout.
• ASIC how-tos: Add "Top-Level ruckus.tcl Structure" section to both Cadence Genus and Synopsys DC how-to guides explaining the manual GenBuildString / AnalyzeSrcFileLists calls required (unlike the Vivado flow).

Files changed

• docs/conf.py — theme, theme options, copyright
• docs/requirements.txt — furo → sphinx-rtd-theme, remove sphinx-autodoc-typehints
• docs/explanation/build_pipeline.rst — make gui section, DFX rename
• docs/explanation/ruckus_tcl_model.rst — annotated recursion diagram
• docs/explanation/output_artifacts.rst — jsmith → smith
• docs/tutorial/first_vivado_build.rst — jsmith → smith
• docs/how-to/cadence_genus.rst — ruckus.tcl structure section
• docs/how-to/synopsys_dc.rst — ruckus.tcl structure section