v6: docs: fix typos, examples, and utility class docs (#42430)

v6: docs: fix typos, examples, and utility class docs (#42430)

Minor documentation fixes across multiple site files:
- examples/sign-in: add meaningful alt text for the logo image for accessibility.
- components/dialog: add missing space in example button markup.
- components/navbar: update navbar example to use new prefix-style responsive class (`.{breakpoint}:navbar-expand`).
- forms/layout: update horizontal form docs to recommend `.{breakpoint}:col-*` usage.
- getting-started/approach: fix grammar/typos, correct docsref path, and clarify component modifier/variant naming (mention `.btn-solid` and `.theme-primary`).
- getting-started/javascript: adjust JS-required components list (add Dialog, remove Accordions/Modals lines).
- guides/migration: fix possessive "its" typos in breakpoint/container notes.
- helpers/stretched-link: correct placeholder title text.
- utilities/background: update example to use `fg-contrast-{name}` / `fg-{name}` classes instead of previous `color-*` naming.

These changes improve clarity, accessibility, and align docs with updated class naming conventions.

v6: docs: fix typos, examples, and utility class docs

text would always be white on black background
Update example code for background gradient usage
Update margin utilities docs for v6 spacers
Docs updated to reflect the v6 spacing scale: sizes now range from 0 through 9 (`.m-0`–`.m-9`) and the multiplier values for sizes 3–9 were adjusted.

Negative margin docs were changed to note that v6 only supports negative inline margins (`.ms--1`, `.me--2`) using `-1` and `-2` spacers and that the v5-style full negative utilities (e.g. `.mt-n1`) were removed.

The responsive examples were updated to list `.{breakpoint}m-0` through `.{breakpoint}m-9` and `.{breakpoint}m-auto`.