Move Help section to the end of the left column

[obulat]

Fixes

Fixes #216 by @panchovm
Fixes #226 by @obulat
Fixes #209 by @panchovm

Description

This PR moves the Help section to the end of the left column to match v2 mockups.

Technical details

This PR is using Vue Vocabulary AppModal component to open Help links in the modals.
Using the Vocabulary component also solves the problem of page jumping up whenever the help link has been clicked.
One point that still needs addressing, probably in the Vocabulary component, is the fact that the body can still be scrolled when a modal is open.

This PR also adds markdown-it package for writing the longer Modal texts in markdown instead of html/plain text. This approach has two advantages:

1. The logic of i18n interpolation becomes more complicated with links and images inside the texts, which is especially problematic for translators. With the previous approach, the translators would get a broken-up string like:

'text': 'Read more about the license at {link}',
'link': 'The Creative Commons site'

With markdown, it becomes:

'text': 'Read more about the licenses at [The Creative Commons site](www.creativecommons.org...)

So, the whole sentence is in one place, instead of being broken up into chunks.

2. We can stop using raw HTML with v-html directive, which might be dangerous with the user-provided text (or translator-provided).

Screenshots

Help Section:

Modal open:

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main or master).
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[dhruvkb]

Better to use an Array when keys are sequential integers?

[obulat]

The reason I didn't want to use an Array, I think, was I wanted the keys to start at 1, not at 0. With an array, I can't check if the modal is open with if modals[openModal], because when the open modal is at index 0, this returns false. I've solved it by checking if openModal !== null instead, and now it looks much cleaner, I think.

[dhruvkb]

You can always create an Array with n+1 elements and leave the index 0 empty.