news: fix images refs

relative paths break when the post is shown in a descendant path

compare:
https://neovim.io/news/
vs
https://neovim.io/news/2021/07

Author: justinmk

_posts/2020-04-14-newsletter.md

@@ -65,7 +65,7 @@ As always, check [Following HEAD](https://github.com/neovim/neovim/wiki/Followin
 
 
  - New flag `msgsep` for `display` to avoid fullscreen scrolling for messages (**@bfredl**, [#8088](https://github.com/neovim/neovim/pull/8088))
-   - See it in action: ![Demonstration of the new multiline message functionality](../images/news9_msgsep.gif)
+   - See it in action: ![Demonstration of the new multiline message functionality](/images/news9_msgsep.gif)
 
  - Numbered marks are now saved in shada files (**@ZyX-I**, [#5908](https://github.com/neovim/neovim/pull/5908))
 
@@ -102,7 +102,7 @@ As always, check [Following HEAD](https://github.com/neovim/neovim/wiki/Followin
 - Channels: support buffered output and bytes sockets/stdio (**@bfredl**, [#6844](https://github.com/neovim/neovim/pull/6844))
 
 - A proper viml expression parser is now available (**@ZyX-l**, [#7234](https://github.com/neovim/neovim/pull/7234))
-  - It comes with various highlight groups, have a look at the following screen recording: ![Demonstration of expression highlighting](../images/news9_expression_highlight.gif)
+  - It comes with various highlight groups, have a look at the following screen recording: ![Demonstration of expression highlighting](/images/news9_expression_highlight.gif)
 
 
 ### The near future: Upcoming

_posts/2021-07-12-newsletter.md

@@ -127,10 +127,9 @@ Of course, these were not the only [major changes in 0.5](https://github.com/neo
 
 There is an improved decoration provider API that allows setting and interacting with extmarks (invisible anchored text markers that move when surrounding text is edited), virtual text (text overlays that can now be drawn at any position on screen), and highlights (which is heavily leveraged by nvim-treesitter).
 
-The following notification mockups taken from a [Discourse post](https://neovim.discourse.group/t/wip-animated-notifications-plugin/448) by [@sunjon](https://github.com/sunjon) show what can be achieved with this API in combination with LuaJIT:
+This mockup from a [post](https://neovim.discourse.group/t/wip-animated-notifications-plugin/448) by [@sunjon](https://github.com/sunjon) shows what can be achieved with this API:
 
-![Notification mockup 1](../images/news11_notif1.gif)
-![Notification mockup 2](../images/news11_notif2.gif)
+![Notification mockup 1](/images/news11_notif1.gif)
 
 ### Floats