make markdown root folder configurable

[PrajwolAmatya]

Description

With this PR the markdown root folder can be made configurable. Previously all the presentation markdown files were supposed to be inside the markdown directory.
With the addition of new env PRESENTATIONS_ROOT, the root folder for all the presentation markdown files can be configured.

Usage:

1. make copy of .env.example and name it .env
2. set the value of PRESENTATIONS_ROOT=<markdown-directory-path>

The folder structure will be as below:

<repo-name>
└── PRESENTATIONS_ROOT
    ├── new-presentation
    │   ├── new-presentation.md
    │   ├── new-presentation-image-01.png
    │   └ ...      
    ├── demo
    │   ├── demo.md
    │   ├── demo-image-01.png
    │   └ ...      
    └ ...
    └ config.yml

Added env on training repo as well: https://github.com/opf/training/pull/138

Related WP:
https://community.openproject.org/projects/revealjs/work_packages/60523/activity

[nirajacharya2]

node should have a process object to get envs from
process.env.PRESENTATIONS_ROOT

[PrajwolAmatya]

For this the env should be exported. IMO, it would be better to read from .env file directly rather than exporting the variable.

[individual-it]

Where will this setting be documented?

[PrajwolAmatya]

Where will this setting be documented?

I have updated the docs. 👍