flutter: update page; flutter-pub: add page (#10734)

* [flutter] added flutter-pub main commands

* Update flutter-pub.md

* Update flutter-pub.md

* Update flutter-pub.md

* Update pages/common/flutter-pub.md

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

* Update pages/common/flutter-pub.md

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

* Update pages/common/flutter-pub.md

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

* Removing unnecessary commas

* Update pages/common/flutter-pub.md

Co-authored-by: Lena <126529524+acuteenvy@users.noreply.github.com>

---------

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: Jack Lin <blueskyson1401@gmail.com>
Co-authored-by: Lena <126529524+acuteenvy@users.noreply.github.com>

pages/common/flutter-pub.md

@@ -0,0 +1,21 @@
+# flutter pub
+
+> Flutter's package manager.
+> Note: Packages are available on <https://pub.dev>. See also: `flutter`.
+> More information: <https://docs.flutter.dev/packages-and-plugins/using-packages>.
+
+- Download/Update all packages specified in `pubspec.yaml`:
+
+`flutter pub get`
+
+- Add a package dependency to an app:
+
+`flutter pub add {{package1 package2 ...}}`
+
+- Remove a package dependency from an app:
+
+`flutter pub remove {{package1 package2 ...}}`
+
+- Upgrade to the highest version of a package that is allowed by `pubspec.yaml`:
+
+`flutter pub upgrade {{package}}`