Format todos as to-do items / To-Do List

Why is this change being made?

The issues gitlab-org/technical-writing#252 (closed) and gitlab-org/technical-writing#252 (closed) summarize the proper usage and current inconsistencies around GitLab's To-Do List feature and its to-do items.

This MR fixes:

• The spelling on the Features page - so it's no longer Todos, but To-Do List, to follow how it's referred to in the docs.
• Image in the Features page - so it's up to date and uses fake user information.
• References to the feature in Handbook pages.

📣 FYI I haven't edited todos in categories.yml, because it's not really user-facing, and I'm not sure what the technical consequences would be.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)