PR: Update PR template to link to developer COI and clarify other text

[CAM-Gerlach]

Pull Request Checklist

• Read and followed this repo's Contributing Guidelines
• Based your PR on the latest version of the correct branch (master or 3.x)
• Ensured your pull request hasn't eliminated unrelated blank lines/spaces,
  modified the spyder/defaults directory, or added new icons/assets
• Described your changes and the motivation for them below

Affirmation

By submitting this Pull Request, I affirm that:

• I release the content of this PR under Spyder's MIT (Expat) license.
• I hold the copyright to all the content in this PR, have not copied any of it
  directly or indirectly from any source other than the public domain,
  and no other party owns the rights to this work.

I affirm all of the above: CAM-Gerlach

Description of Changes

Updates the pull request template to simply link to the standard and accepted Developer COI used by many open source projects, rather than lengthier bespoke legal text included previously. Also does some minor cleanup, refinements and clarification to the template text.

[ccordoba12]

Thanks!