Restructure Test category docs: split long articles and reorder sections

[catpetrychenko-rgb]

Why

• Current articles are too big. The largest one, test-case-creation-and-editing.md, is 527 lines
  and covers eight different tasks on a single page (creation, steps, attachments, priorities,
  parameters, etc.).
• In such a solid wall of text it's hard to quickly find the one thing you came for.
• Suite and folder basics currently sit at the end, in other-features-for-test-case-design,
  although a test lives inside a suite - it's more logical to first show where tests go,
  and only then how to create them.

Approach

• One article = one task. Six large pages become a set of smaller ones.
• Сontent is reformatted and moved into a more convenient structure.
• Advanced actions (PDF export, sharing) stay at the end, as they are operations on top of
  an already existing structure.
• Add new screenshots and diagrams; reserve placeholders for future videos.

Possible structure (draft)

1. Concepts - Classical, BDD, the difference, conversion
2. Editors - Classical, BDD, Test steps reference
3. Suites and folders - what it is, how to create, test tree, convert suite to folder
   (moved here from other-features-for-test-case-design)
4. Create and edit a test - will be taken from test-case-creation-and-editing
5. Organize tests - copy / duplicate / move / restore - will be taken from copy-and-move-your-tests
6. Manage and share - bulk select, URL/ID, tree display, PDF export, sharing -
   will be taken from other-features-for-test-case-design