Enhance accessibility-ready guideline for Skip to Content Link #372

• I've checked the existing issues for this change

URL
page URL at https://wpaccessibility.org/docs/accessibility-ready/theme-guidelines/skip-to-content/

Title
Skip to Content Link

What Needs Editing?
Add info with classic vs. block theme information and examples how to achieve the guidelines.

Related content in the Knowledge base:

• https://wpaccessibility.org/docs/topics/code/components/skip-links/
• https://wpaccessibility.org/docs/topics/code/focus-handling/tabindex/
• https://wpaccessibility.org/docs/testing/keyboard/

Additional context
Tracking issue: #332