fix(docs): update sidebar link color to meet WCAG AA contrast requirements (#814)

## Summary

Fixes the left sidebar navigation link color that fails WCAG AA
accessibility contrast requirements.

## Problem

The `.menu__link` rule in the Docusaurus custom CSS used `color:
#e6e6e6` (light grey) for both light and dark themes. On the white
light-mode background (`#ffffff`), this produces a contrast ratio of
~1.45:1 — far below the WCAG AA minimum of 4.5:1 for normal text.

## Changes

- **Light mode**: Changed `.menu__link` color from `#e6e6e6` to
`var(--ifm-font-color-base)` (`#161616`), yielding ~13:1 contrast ratio
against white — exceeds WCAG AAA (7:1).
- **Dark mode**: Added `[data-theme='dark'] .menu__link` rule preserving
`#e6e6e6`, which provides ~12:1 contrast against the dark background
(`#1f1f1f`).

## File Changed

- `docs/docusaurus/src/css/custom.css`

Fixes #813

Co-authored-by: Bill Berry <wbery@microsoft.com>

Author: WilliamBerryiii

docs/docusaurus/src/css/custom.css

@@ -191,7 +191,7 @@ a:visited {
   font-weight: 400;
   line-height: 25.6px;
   padding: 2px 0 2px 16px;
-  color: #e6e6e6;
+  color: var(--ifm-font-color-base);
 }
 
 .menu__link:hover {
@@ -208,6 +208,10 @@ a:visited {
   background-color: var(--ms-learn-section-bg);
 }
 
+[data-theme='dark'] .menu__link {
+  color: #e6e6e6;
+}
+
 .menu__list-item-collapsible > .menu__link {
   font-weight: 400;
 }