Rename `continuous_time_mcs` into the `lecture-` family

[mmcky]

continuous_time_mcs is a live lecture series (Continuous Time Markov Chains) sitting outside the lecture- prefix, and its underscores violate the existing dash-not-underscore rule. Under the draft repository-naming standard (QEP-3, discussion at QuantEcon/qeps#7) it is a rename candidate, natural to batch with the core-series renames in #333.

Suggested target: lecture-continuous-time-mcs (team to confirm the topic token).

Scope constraint — repository-rename-only, same as #333: no hosting, domain, or Pages changes. GitHub auto-redirects the old repo URL and git remotes.

Checklist (mirrors #333's per-repo list):

• Confirm the target name
• Rename the repo in Settings
• Update in-repo references (publish.yml release-asset name, _config.yml, notebook download links, README badges)
• Verify the site is still serving
• Update inbound links from sibling repos and the QuantEcon website
• Notify the team to update local remotes