Manual: Add sub-directories to methods section

docs/methods/sampling/ehrenfest.md

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+# Ehrenfest Dynamics
+
+Unfortunately, nobody has gotten around to writing this page yet :-(

docs/methods/sampling/index.md

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+# Sampling
+
+```{toctree}
+---
+titlesonly:
+maxdepth: 1
+---
+Molecular Dynamics <https://www.cp2k.org/howto:md>
+metadynamics
+path_integrals
+Surface Hopping with NEWTON-X <https://www.cp2k.org/howto:newtonx>
+i-PI <https://www.cp2k.org/howto:ipi>
+Gibbs Ensemble Monte Carlo <https://www.cp2k.org/howto:gemc>
+Langevin Dynamics <https://www.cp2k.org/howto:langevin_regions>
+ehrenfest
+```

docs/methods/sampling/metadynamics.md

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+# Metadynamics
+
+Unfortunately, nobody has gotten around to writing this page yet :-(

docs/methods/sampling/path_integrals.md

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+# Path Integrals
+
+Unfortunately, nobody has gotten around to writing this page yet :-(