gopls/doc: yet more doc tweaks

- shrink and center the "Split package" image
- rename "Release policy" to "Releases"
- temporarily remove the v0.21.0.md file; it shouldn't appear
  in the v0.20 branch.

Change-Id: Id41d9c43ce071a706bf08baf7402888ea932a48c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/691115
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: adonovan

gopls/doc/index.md

@@ -75,7 +75,7 @@ observe the effect. Each client has its own way to restart the server.
 Learn more in the
 [advanced installation instructions](advanced.md#installing-unreleased-versions).
 
-## Release policy
+## Releases
 
 Gopls [releases](release/) follow [semantic versioning](http://semver.org), with
 major changes and new features introduced only in new minor versions
@@ -120,7 +120,7 @@ full list of `gopls` settings can be found in the [settings documentation](setti
 variables you configure. Some editors, such as VS Code, allow users to
 selectively override the values of some environment variables.
 
-## Support Policy
+## Support policy
 
 Gopls is maintained by engineers on the
 [Go tools team](https://github.com/orgs/golang/teams/tools-team/members),

gopls/doc/release/v0.20.0.md

@@ -32,7 +32,7 @@ component, then visualize the dependencies among the components
 Refresh the page each time you edit your code to see the latest
 information.
 
-<img title="Split package 'fmt'" src="../assets/splitpkg.png">
+<p align="center"><img title="Split package 'fmt'" src="../assets/splitpkg.png" width="600"></p>
 
 The tool makes it easy to iterate over potential decompositions
 until you find one you are happy with. A future version of