Add Plugins Page to microsite (#2144)

* Add Plugins Page to microsite

* Updates to microsite Plugins Page

* Correct plugin authors and documentation links
* Add category to UI
* Move documentation from README to the docs website
* Use external urls for logos

* Updates to microsite Plugins Page

* trailing whitespace
* Run prettier on new markdown

* Updates to microsite Plugins Page

* Move plugins link
* Clarify category field

* Updates to microsite Plugins Page

* Add authorUrl field to plugin config
* Render author as a muted link like those in the site map

* Updates to microsite Plugins Page

* Add authorUrl field to example in docs

* Updates to microsite Plugins Page

* Add npmPackageName field - intended for future use

* Updates to microsite Plugins Page

* Add npmPackageName field - intended for future use

* Updates to microsite Plugins Page

* Use correct docs link

microsite/data/plugins/rollbar.yaml

@@ -0,0 +1,10 @@
+---
+title: Rollbar
+author: '@andrewthauer'
+authorUrl: https://github.com/andrewthauer
+category: Monitoring
+description: View Rollbar errors for your services in Backstage.
+documentation: https://github.com/spotify/backstage/tree/master/plugins/rollbar
+iconUrl: https://rollbar.com/assets/media/rollbar-mark-color.png
+npmPackageName: '@backstage/plugin-rollbar'
+

microsite/data/plugins/sentry.yaml

@@ -0,0 +1,10 @@
+---
+title: Sentry
+author: Spotify
+authorUrl: https://www.spotify.com/
+category: Monitoring
+description: View Sentry issues in Backstage.
+documentation: https://github.com/spotify/backstage/tree/master/plugins/sentry
+iconUrl: https://sentry-brand.storage.googleapis.com/sentry-glyph-white.png
+npmPackageName: '@backstage/plugin-sentry'
+

microsite/data/plugins/travis-ci.yaml

@@ -0,0 +1,10 @@
+---
+title: Travis CI
+author: roadie.io
+authorUrl: https://roadie.io/
+category: CI
+description: View Travis CI builds for your service in Backstage.
+documentation: https://roadie.io/backstage/plugins/travis-ci
+iconUrl: https://roadie.io/static/af2941eaf0af675facb281d566f42e14/45f2b/travis-ci-mascot-200x200.png
+npmPackageName: '@roadiehq/backstage-plugin-travis-ci'
+