docs: add population and EOS posterior guides

[oshaughnessy-junior]

Closes #32

Scope

• add an LDG/Condor-oriented mock-population workflow guide
• add an EOS posterior-pipeline guide with safe current-working-directory semantics
• register both pages in the Sphinx index

Validation

• git diff --check passes
• create_eos_posterior_pipeline --help passes in the clean worktree Pixi environment with PYTHONPATH=MonteCarloMarginalizeCode/Code
• focused strict Sphinx parsing reaches both new pages without page-specific diagnostics; the repository-wide build retains 927 pre-existing autodoc/docutils warnings

Review

Independent adversarial review found and verified the correction of the EOS working-directory behavior. The unrelated local pixi.lock modification is not part of this PR.