added blog post about micro-ROS at ERF 2021

[JanStaschulat]

tested links to pdfs and external links locally with bundle exec jekyll serve.

[JanStaschulat]

output of the htlm checker:

Checking 779 external links...
Ran on 141 files!
- ./_site/docs/concepts/rtos/comparison/index.html
  *  External link https://interactive.freertos.org/hc/en-us/community/posts/210029046-POSIX-Wrapper-for-FreeRTOS failed: 403 No error
/home/runner/work/micro-ROS.github.io/micro-ROS.github.io/vendor/bundle/ruby/2.7.0/gems/html-proofer-3.15.1/lib/html-proofer/runner.rb:172:in `print_failed_tests': HTML-Proofer found 1 failure! (RuntimeError)

but the external link https://interactive.freertos.org/hc/en-us/community/posts/210029046-POSIX-Wrapper-for-FreeRTOS is correct.
@pablogs9 any idea?

[pablogs9]

I have just updated the URL to a more representative FreeRTOS + POSIX webpage

[ralph-lange]

Please embed the slides and videos directly in the post:

• Slides can be embedded with <embed src="/download/thefilename.pdf" type="application/pdf" width="100%" height="450px"/>, see https://github.com/micro-ROS/micro-ROS.github.io/blame/master/_posts/2020-03-03-micro-ROS-at-ERF2020.md#L16.
• The Share button at YouTube allows getting the HTML snippet to embed the video.

[JanStaschulat]

Please embed the slides and videos directly in the post:

* Slides can be embedded with `<embed src="/download/thefilename.pdf" type="application/pdf" width="100%" height="450px"/>`, see https://github.com/micro-ROS/micro-ROS.github.io/blame/master/_posts/2020-03-03-micro-ROS-at-ERF2020.md#L16.

* The Share button at YouTube allows getting the HTML snippet to embed the video.

thanks for the feedback. It is updated in commit 48d58df

[JanStaschulat]

I updated the blog post accordingly. However, I am not authorized to merge this pull request. @pablogs9 or @ralph-lange, could you merge it?