Suggested changes to Product Designer Onboarding

This MR proposes several changes to the new product designer onboarding issue template.

It can be difficult to get a sense of the proposed version of the template from the diffs; here's a link to the version of the template in this MR. Note that you may want to check the raw on that to see comments in the Markdown.

Proposed changes

Note that the following list is not in any particular order, but items are numbered for ease of reference.

1. Remove tasks that are duplicative of tasks that appear in the org-wide new employee onboarding task (the entire "Company" section has been removed here)
2. Make it easier to find-and-replace key user handles, dates and stage information when creating/setting up a new onboarding issue, by using consistent all-caps-snake-case placeholders in the template
3. Fix inconsistent use of checklists to make them more action-oriented
   1. Some todo items were previously plain unordered list items, they have been made into checklist items
   2. Some formerly checklist items are not todos for the new employee, and the have been made into normal unordered list items
   3. A new "Tips and tricks" section has been introduced at the end of the template to collect some random info list items from various sections
4. Move the optional Product info videos section to the end, after required items
5. Add a "How to use this onboarding issue" section at the start of the document, which heavily recycles information that was previously housed in a "Closing thoughts" section at the end of the template
   1. The new team member will not go through this entire list all at once, so the concept of "closing thoughts" doesn't make much sense -- especially since the content in that section is beneficial for the new team member to access before starting the checklist
6. Remove the task to join a "UX Hangout" session. These seem to no longer happen for all employees? The only one I saw on the calendar is before most US employees' work day would begin.
7. Fix some usage and punctuation
   1. The text of a list item only needs periods if the list item contains more than one sentence
   2. Usages like "Fortnightly" are regional
   3. Slack channel names preceded by a # should be in an inline code block
   4. Headers in Markdown should always be surrounded by blank lines
   5. Lists in Markdown should always be surrounded by blank lines
   6. There should only be one H1 in any Markdown file
8. Change some "[bold text]:" usages before unordered lists within sections to H3s for better semantic organization
9. break some long todos up with sub-bullets to make relevant contextual information easier to digest
10. Add a "What is DevSecOps" item in the "Tech knowledge to know" section
11. Make the "Your onboarding experience" section more actionable and clarify that it is a section about feedback from the new team member about their onboarding experience
12. Add a UX Buddy task to invite the new team member to shadow them in their third week (since that expectation is set for the new team member)
13. Remove the "team chart" link, as it is a dead end
14. Using a line-final whitespace to force newline is fragile and could lead to the slash commands breaking. This edit inserts blank lines between each of the templated slash commands to make sure they don't accidentally get combined into a single line and ignored.

Assigning my manager and onboarding buddies as reviewers here.