Fix more internal documentation links when viewing on GitHub

Applies fixes done on #142 on more links

docs/README.md

@@ -22,7 +22,7 @@ tasks:
 
 And call it by running `task hello` from you terminal.
 
-The above example is just the start, you can take a look at the [usage](usage)
+The above example is just the start, you can take a look at the [usage](usage.md)
 guide to check the full schema documentation and Task features.
 
 ## Features
@@ -34,7 +34,7 @@ guide to check the full schema documentation and Task features.
   to install on your CI script and you're done to use Task as part of your CI pipeline;
 - Truly cross-platform: while most build tools only work well on Linux or macOS,
   Task also supports Windows thanks to [this awesome shell interpreter for Go][sh];
-- Great for code generation: you can easily [prevent a task from running](usage#prevent-unnecessary-work)
+- Great for code generation: you can easily [prevent a task from running](usage.md#prevent-unnecessary-work)
   if a given set of files haven't changed since last run (based either on its
   timestamp or content).
 

docs/_sidebar.md

@@ -1,8 +1,8 @@
-- [Installation](installation)
-- [Usage](usage)
-- [Taskfile Versions](taskfile_versions)
-- [Examples](examples)
-- [Releasing Task](releasing_task)
-- [Alternative Task Runners](alternative_task_runners)
-- [Sponsors and Backers](sponsors_and_backers)
+- [Installation](installation.md)
+- [Usage](usage.md)
+- [Taskfile Versions](taskfile_versions.md)
+- [Examples](examples.md)
+- [Releasing Task](releasing_task.md)
+- [Alternative Task Runners](alternative_task_runners.md)
+- [Sponsors and Backers](sponsors_and_backers.md)
 - [![Github](https://icongram.jgog.in/simple/github.svg?color=808080&size=16)Github](https://github.com/go-task/task)

docs/taskfile_versions.md

@@ -143,6 +143,6 @@ includes:
 
 Please check the [documentation][includes]
 
-[output]: usage#output-syntax
-[ignore_errors]: usage#ignore-errors
-[includes]: usage#including-other-taskfiles
+[output]: usage.md#output-syntax
+[ignore_errors]: usage.md#ignore-errors
+[includes]: usage.md#including-other-taskfiles