Add prev/next buttons to vs-code extension docs

[sm1990]

Description

$subject
e.g. https://ballerina.io/learn/get-started/

Related website/documentation area

Add/Uncomment the relevant area label out of the following.

Describe the problem(s)

A detailed description of the purpose of this improvement.

Describe your solution(s)

A detailed description of how this improvement will solve the problem described above.

Related issue(s) (optional)

Any related issues such as sub tasks and issues reported in other repositories (e.g., component repositories), similar problems, etc.

Suggested label(s) (optional)

Optional comma-separated list of suggested labels. Non committers can’t assign labels to issues, and thereby, this will help issue creators who are not a committer to suggest possible labels.

Suggested assignee(s) (optional)

Optional comma-separated list of suggested team members who should attend the issue. Non committers can’t assign issues to assignees, and thereby, this will help issue creators who are not a committer to suggest possible assignees.