[web] Added hydro-mechanics benchmarks.

web/config.toml

...ks/hydro-mechanics/HM_confined_compression_analytical.png

...ro-mechanics/HM_confined_compression_simulation_error.png

web/content/docs/benchmarks/hydro-mechanics/hm-linear.md

...s/benchmarks/hydro-mechanics/hm-unconfined-compression.md

...nchmarks/hydro-mechanics/lie-hm-linear-single-fracture.md

...content/docs/benchmarks/hydro-mechanics/mesh_UC_final.png

...nt/docs/benchmarks/hydro-mechanics/num_ana_1s_refined.png

...mechanics/transient_validation_unconfined_compression.png

...s/benchmarks/hydro-mechanics/verification_1000s_new-1.png

web/content/docs/benchmarks/hydro-mechanics/x_b_t500.png

web/content/docs/benchmarks/hydro-mechanics/x_p_t500.png

web/themes/ogs/archetypes/.gitkeep