Tile API Update

[ntno]

CSS Refactor

• moved tile grid specific CSS to separate file
• center image within tile

Tile Grid API Update

Tile API

removed image width/height attributes

• img_width removed; use tile_css instead
• img_height removed; use tile_css instead

replaced title attributes with new tooltip attribute

• img_title -> tooltip
• link_title -> tooltip

simplify alternate text specification, use alt_text to describe image or link

• img_alt -> alt_text
• link_text -> alt_text

Documentation Updates

• added alt text guidance
• added links to additional resources on writing alt text
• note that alt text should be a link description for a linked image tile (see discussion in review accessibility of linked image tile #107)
• updated tile examples

Test Updates

• unit tests updated

Checklist

• I have added documentation for new/changed functionality in this PR or in mkdocs-terminal/documentation
• All active GitHub checks for tests, formatting, and security are passing