Create useful default GitHub project files

[seanbudd]

For each of the following, consider if a separate file is needed for the template vs the add-on repos themselves.
e.g. a contributing.md file might need to be different from the template to the add-on repos, but the code of conduct file can probably be shared as a useful default for both.

• Fixup the readmes: Separate repository documentation from add-on help file #31
• Contributing.md
• A code of conduct file
• CodeQL scanning config file
• pull request template
• issue templates
• sponsorship (funding.yml) (do we want to sponsor maintenance of this template? we can set up this repo to encourage sponsorship of https://github.com/nvdaaddons and encourage authors to use a list to sponsor both their project, the add-on template community maintenance and/or NV Access )

[makhlwf]

I would like to work on this, starting with the issue templates. Should the templates be Markdown files, or the YAML templates used recently by the NVDA repository? I guess the latter would be preferred. I'll take some inspiration from existing NVDA issue templates anyway, since they are mostly somewhat relevant.