docs: right-align table numbers and merge RCPSP config tables

- Add docs/_static/custom.css: CSS to right-align data cells in
  benchmark tables (.numtable) and left-align first two columns
  in RCPSP tables (.rcpsp-table) for HTML output
- Update docs/conf.py: add '_static' to html_static_path and
  html_css_files = ['custom.css']
- Add '.. tabularcolumns::' directive before every list-table in
  bench.rst for right-aligned columns in PDF/LaTeX output
- Merge the 8 RCPSP config tables (A/B/C/D × CPython/PyPy) into
  2 tables (CPython and PyPy) with additional Resources and Prec
  columns, reducing repetition and enabling easy cross-config comparison

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: h-g-s

docs/_static/custom.css

@@ -0,0 +1,7 @@
+/* Right-align data cells in benchmark tables */
+table.numtable td { text-align: right; }
+table.numtable th { text-align: center; }
+
+/* Left-align the first two columns in RCPSP tables (Resources, Prec) */
+table.rcpsp-table td:nth-child(1),
+table.rcpsp-table td:nth-child(2) { text-align: left; }