remove indentation in all code blocks in markdown files

We use padding for first level indentation in code block since this PR got merged.

For example consider this codeblock:

```dvc
    $ pip install gitpython
    $ pip install git+git://github.com/iterative/dvc
```

Output

Expected Output

This can be fixed by removing the unnecessary 4 spaces indentation we have given under every code block. Since there are a lot of files, I recommend a smart way to fix it. For example a bash script could be made to run over all .md files and remove the indentation. Keep in mind that we don’t have to completely remove indentation. Just the first level. For example:

```js
    function sum(a, b) {
        return a+b
    }
```

converts to:

```js
function sum(a, b) {
    return a+b
}
```

not this:

```js
function sum(a, b) {
return a+b
}
```

[shcheklein]

hi, @ryokugyu ! That would be great. Thank you. I’ll assign it to myself to lock the issue. Feel free to proceed with it.

[shcheklein]

Fixed by @ryokugyu .