Clarify differences between Presentation and Content MathML

AI Disclosure: This issue was drafted with the assistance of an AI language model.

Is your feature request related to a problem? Please describe.
The current README does not clearly explain the differences between Presentation MathML (visual rendering) and Content MathML (semantic meaning), nor which subsets are supported. This may causes confusion for users who want to:

• Generate MathML for browsers.
• Parse existing MathML documents.
• Understand the limitations of MathML.jl.

Describe the solution you’d like
Update the README to:

1. Add a "MathML Subsets" section with a table of supported/unsupported tags.
2. Include comparative examples for Presentation vs. Content MathML.
3. Add a FAQ section addressing common questions (e.g., "Why doesn’t MathML.jl support Presentation MathML?").

Describe alternatives you’ve considered

• Relying on external documentation (e.g., W3C specs).
  → Problem: External docs don’t cover MathML.jl’s specific limitations.

Additional context

• Priority: ⭐⭐⭐ (Low effort, high impact for usability).

Start with the README and inspect MathML.jl's parser behavior to verify which Presentation and Content MathML tags are supported. Document the confirmed subsets in a table, add comparative examples and the requested FAQ, and ensure the README accurately explains the project's limitations for generation and parsing.