Migrating from Travis to GitHub Actions (#16322)

* Adds Travis CI migration guide

* Updated variable syntax

* Updated links

* Fixed variable

* Split into two tables to fix rendering

* Removed section about upstream, might not be applicable here

* Made formatting more consistent

* Added misc edits, "Next steps" section, matrix example

* Added small edits

* Added updates from review

* Edited "Key similarities" intro

* Replaced diagram with link to "Learning GitHub Actions"

* Moved links into a "Before you begin" section

* Added "Parallel job processing"

* Added small edits, "Default environment variables" section

* Added "Targeting specific branches"

* Added "Checking out submodules"

* Moved into nested subsection

* Updated "Assumptions about language dependencies"

* Renamed section

* Adds some small edits

* Moved all headings down 1 level

* Apply suggestions from code review

Co-authored-by: Lucas Costi <lucascosti@users.noreply.github.com>

* Added updates from review

* Added updates from review

* Fixed rendering, added small edit

* Removed Travis concepts paragraph, as it seems superfluous

* Added minor edits

* Reframed the "differences" section to rather emphasize Actions features that might distinguish it from others

* Reframed additional differences to emphasize Actions capabilities

* Small edit

* More small edits

* Apply suggestions from code review

Co-authored-by: Lucas Costi <lucascosti@users.noreply.github.com>

* Revised conditional example

* Revised build matrix exmple

* Updated python example

* Updated examples

* Changed example to "Building with Node.js"

* Adds fixes from peer review

* Added minor edits

* Added some finishing touches

* Update migrating-from-travis-ci-to-github-actions.md

* Small clarification

Co-authored-by: Lucas Costi <lucascosti@users.noreply.github.com>

Author: martin389

assets/images/help/images/comparing-travis-with-actions.png

@@ -36,7 +36,8 @@ versions:
 {% link_with_intro /managing-complex-workflows %}
 {% link_with_intro /sharing-workflows-with-your-organization %}
 {% link_with_intro /security-hardening-for-github-actions %}
+{% link_with_intro /migrating-from-azure-pipelines-to-github-actions %}
 {% link_with_intro /migrating-from-circleci-to-github-actions %}
 {% link_with_intro /migrating-from-gitlab-cicd-to-github-actions %}
-{% link_with_intro /migrating-from-azure-pipelines-to-github-actions %}
 {% link_with_intro /migrating-from-jenkins-to-github-actions %}
+{% link_with_intro /migrating-from-travis-ci-to-github-actions %}