[lizavab]

Update the Protected Areas 2021 and 2024 Nomenclature and Mapping Guideline in DOCS/Protected_Areas, first added in #79.

Content changes:

• Restructured section headings from Roman-numeral plain text (I. Introduction) to Markdown headings, fixing the document's table of contents / indexing
• Preserved the source document's LC/LU nomenclature numbering by hardcoding the class codes (e.g. 1. Urban, 1.1 Urban fabric, 2. Cropland) in the heading text and marking them {.unnumbered} (81 headings)
• Corrected category from products to guidelines
• Added a list of abbreviations
• Fixed the footnote definitions (previously 2 orphaned definitions; now 33 definitions / 33 references)
• Re-uploaded several incorrect figures (20 corrected, 369 in total)