docs: Improve headings for changelog.

docs/overview/changelog.md

@@ -2,7 +2,9 @@
 
 All notable changes to the Zulip server are documented in this file.
 
-## 4.0 -- Unreleased
+## Zulip 4.x series
+
+### 4.0 -- Unreleased
 
 This section lists notable unreleased changes; it is generally updated
 in bursts.
@@ -56,6 +58,8 @@ in bursts.
 - Updated webapp codebase to use many modern ES6 patterns.
 - Relabeled :smile: and :stuck_out_tongue: emoji to use better codepoints.
 
+## Zulip 3.x series
+
 ### 3.2 -- September 15, 2020
 
 - Switched from `libmemcached` to `python-binary-memcached`, a
@@ -319,6 +323,8 @@ in bursts.
   system was always a hack, was only ever used for one endpoint, and
   did not provide a measureable latency benefit over HTTP/2.
 
+## Zulip 2.1.x series
+
 ### 2.1.7 -- 2020-06-25
 
 - CVE-2020-15070: Fix privilege escalation vulnerability with custom
@@ -669,6 +675,8 @@ lose the setting and need to re-enable it.
 - Enabled webpack code splitting and deduplication.
 - Started migrating our frontend codebase to TypeScript.
 
+## Zulip 2.0.x series
+
 ### 2.0.8 -- 2019-12-12
 
 - CVE-2019-19775: Close open redirect in thumbnail view.
@@ -839,6 +847,8 @@ and is enabled by default in that case. To disable it, set
 - Countless behind-the-scenes improvements to Zulip's codebase,
   tooling, automated tests, error handling, and APIs.
 
+## Zulip 1.9.x series
+
 ### 1.9.2 -- 2019-01-29
 
 This release migrates Zulip off a deprecated Google+ API (necessary
@@ -992,6 +1002,8 @@ Zulip installations; it has minimal changes for existing servers.
 - Our steady work on codebase quality and our automated test suite
   continues.  Backend test coverage is now an incredible 98%.
 
+## Zulip 1.8.x series
+
 ### 1.8.1 -- 2018-05-07
 
 - Added an automated tool (`manage.py register_server`) to sign up for
@@ -1202,6 +1214,8 @@ Zulip installations; it has minimal changes for existing servers.
 
 This major release has no special upgrade notes.
 
+## Zulip 1.7.x series
+
 ### 1.7.2 -- 2018-04-12
 
 This is a security release, with a handful of cherry-picked changes
@@ -1424,6 +1438,8 @@ running a version from before 1.7 should upgrade directly to 1.7.1.
 - Removed several obsolete settings.
 - Partially completed migration to webpack as our static asset bundler.
 
+## Zulip 1.6.x and older
+
 ### 1.6.0 -- 2017-06-06
 
 #### Highlights