Doc-version landing pages need rework

[chalin]

On desktop, readers have direct access to the following links on every documentation page:

Most doc-version landing pages have a sentence of the form "Improvements to these docs are encouraged through pull requests to the etcd project on GitHub", with a link to the etcd repo -- from the days where the docs were in that repo. Given the GH issue links on each page, let's drop that sentence from the opening paragraph of each doc-version landing page.

• v2.4: drop "Improvements to these docs are encouraged through pull requests to the etcd project on GitHub".
• v3.1: drop "Improvements to these docs are ... project on GitHub".
• v3.2: drop "Improvements to these docs are ... project on GitHub".
• v3.3: add a paragraph so that it matches the (edited) v3.4 page's opening text.
• v3.4: drop "Improvements to these docs are ... project on GitHub".
• v3.5:
  • Drop "Improvements to these docs are ... project on GitHub"
  • Simplify the first sentence to match v3.4. (The word DRAFT is prevalent enough on the page, we don't need another mention of, say, "upcoming" in the opening sentence.) This will also make it easier when creating the v3.5 directory.