fix(docs): create a new page for HOWTOs sidebar section

Rename "How tos" to "HOWTOs", I find "How tos" as difficult to read.

Add a new page `docs/site/Howtos.md` and use it to back "HOWTOs"
section. Before this change, both "HOWTOs" and "Key concenpts" were
sharing the same section page, which was confusing.

docs/site/How-tos.md

@@ -0,0 +1,22 @@
+---
+lang: en
+title: 'HOWTOs'
+keywords: LoopBack 4.0, LoopBack 4
+sidebar: lb4_sidebar
+permalink: /doc/en/lb4/How-tos.html
+---
+
+We provide the following "how to" guides:
+
+- [Exposing GraphQL APIs](exposing-graphql-apis.md): a tutorial showing how to
+  expose GraphQL APIs in an existing LoopBack application.
+
+- [Deploying to IBM Cloud](Deploying-to-IBM-Cloud.md): a basic guide for
+  deploying LoopBack 4 applications to IBM Cloud using Cloudfoundry.
+
+- [Serving static files](Serving-static-files.md): a guide for setting up static
+  assets (e.g. HTML5 front-end).
+
+- [Database migrations](Database-migrations.md): a tutorial showing how to
+  automatically update the database schema to match the models defined by the
+  application.

docs/site/sidebars/lb4_sidebar.yml

@@ -191,8 +191,8 @@ children:
     url: Error-handling.html
     output: 'web, pdf'
 
-- title: 'How tos'
-  url: Concepts.html
+- title: 'How-tos'
+  url: How-tos.html
   output: 'web, pdf'
   children:
     - title: 'Exposing GraphQL APIs'