Add community engagement links

[ahouseholder]

This PR:

• resolves Make it more obvious how to contribute for someone looking at web docs #461
• adds issue templates for bug reports, feature requests, and questions
• adds docs/_includes/helping_out.md with a grid of options (see screenshot below), including Ask a question, Report a problem, Suggest an improvement, Join the conversation, See what we're working on, and Get more involved.
• Each of those grid cards contains one or more links to the appropriate items. The use of issue templates lets us have the Ask, Report, and Suggest items all link to the new issue form but with the appropriate template included. Join goes to Discussions. See is just links to existing issues and PRs, and Get links to the project wiki.
• using the same icons from the grid cards, there are more icons in the page footer for the entire site that lead to creating issues and discussions. I deliberately did not include the See or Get icons since those seemed less relevant to appear on every page.
• Removed the Issue Tracker link from the header. Now that we have the templates linked from the icons in the footer, this seemed redundant (and it took up a lot of horizontal space in the header nav)
• The docs/_includes/helping_out.md file is included in three places within the site: (1) on the home page, (2) on the contributing page (which is renamed to community engagement in the navigation), and (3) on the contact us page.

Screenshot of docs/_includes/helping_out.md:

Screenshot of new icons in page footer. Each has tool-tips indicating what they are, consistent with the grid cards above:

Incidental changes also included

• Coincident with these changes, I copied some of the content of the original CONTRIBUTING.md file into the project wiki. That part is already done, this PR just removes the now-redundant content from CONTRIBUTING.md
• Copy edit of docs/about/index.md to be less specific about SSVC versions so we don't have to keep updating it every time we release something
• Reorganize both the list in docs/about/index.md and the site nav so that the about subsections are consistently ordered

Note that this PR is specifically not addressing #401, which is more focused on creating content in the wiki that is more focused on "github usage tips".

[j---]

Love this.