Fix broken links in documentation

Close mmistakes#3004

Author: mmistakes

CHANGELOG.md

@@ -1,5 +1,10 @@
 ## Unreleased
 
+### Bug Fixes
+
+- Fix broken sidebar image in sample post. [#3013](https://github.com/mmistakes/minimal-mistakes/issues/3013)
+- Fix broken links in Upgrading documentation. [#3004](https://github.com/mmistakes/minimal-mistakes/issues/3004)
+
 ### Enhancements
 
 - Remove `h2` from skip links navigation as it is not important for site structure. [#3012](https://github.com/mmistakes/minimal-mistakes/pull/3012)

docs/_docs/04-upgrading.md

@@ -2,11 +2,11 @@
 title: "Upgrading"
 permalink: /docs/upgrading/
 excerpt: "Instructions and suggestions for upgrading the theme."
-last_modified_at: 2018-11-25T19:38:20-05:00
+last_modified_at: 2021-06-23T08:15:34-04:00
 toc: true
 ---
 
-If you're using the [Ruby Gem]({{ "/docs/quick-start-guide/#ruby-gem-method" | relative_url }}) or [remote theme]({{ "/docs/quick-start-guide/#github-pages-method" | relative_url }}) versions of Minimal Mistakes, upgrading is fairly painless.
+If you're using the [Ruby Gem]({{ "/docs/quick-start-guide/#gem-based-method" | relative_url }}) or [remote theme]({{ "/docs/quick-start-guide/#remote-theme-method" | relative_url }}) versions of Minimal Mistakes, upgrading is fairly painless.
 
 To check which version you are currently using, view the source of your built site and you should see something similar to:
 

docs/_docs/18-history.md

@@ -5,12 +5,17 @@ permalink: /docs/history/
 excerpt: "Change log of enhancements and bug fixes made to the theme."
 sidebar:
   nav: docs
-last_modified_at: 2021-06-23T08:12:46-04:00
+last_modified_at: 2021-06-23T08:17:10-04:00
 toc: false
 ---
 
 ## Unreleased
 
+### Bug Fixes
+
+- Fix broken sidebar image in sample post. [#3013](https://github.com/mmistakes/minimal-mistakes/issues/3013)
+- Fix broken links in Upgrading documentation. [#3004](https://github.com/mmistakes/minimal-mistakes/issues/3004)
+
 ### Enhancements
 
 - Remove `h2` from skip links navigation as it is not important for site structure. [#3012](https://github.com/mmistakes/minimal-mistakes/pull/3012)