Display properties of .md file in preview

## Problem

I want to see properties of spec in a single click from 'Specs' pane without necessity to open sources of Markdown file.

## Proposed solution

Something like

## Alternatives considered

There are other possible layouts for that and different possible considerations on whether which data types have to be stored.

The view can be either read-only or read-write. Read-write will require YAML serialization and will cause round-trip problems for any unusual YAML put to properties.

## Additional context

Start by locating the Specs pane and the preview entry point for Markdown files. Review the proposed layout and decide which properties are currently available before implementing the single-click view. Done means a spec's properties are visible from the Specs pane without opening its source, with the chosen read-only or read-write behavior documented by the implementation.

Written by the indexing model from the issue text.