As a followup to #2801 , this PR:

• Adds and updates background, clarifications and links to clearly reflect the current status quo situation with Windows support policies, specifically that the lifecycle refereed to is the Fixed lifecycle policy, clarifying the distinction with the newer "Modern" lifecycle policy introduced since the PEP's last revision in this area, and prepares for a future decision on the policy to adopt toward Windows 10+ releases which follow the latter (without intending to make a substantive policy change itself, just clarifying the present status quo until a more specific policy is discussed, decided and adopted).
• Further clarifies the situation with ESU updates, and improves the text and terminology used therein.
• Makes several minor technical fixes to PEP 11, removing the obsolete headers and footer and updating and inlining the last remaining footnote-style link.

@zooba My intent here is to avoid any substantive change to the existing CPython policy or declaration of a new policy for Windows 10+ in this PR, and only update and clarify the existing status quo on Microsoft's current lifecycles. To that end, I've kept the same wording wrt CPython's support policy, aside from a few very minor textual clarifications. If you prefer, I could add an explicit statement that the support policy for Windows 10+ is yet to be conclusively decided, with perhaps some reasonable lower and upper bounds, or other tweaks if you think them prudent.