Tweaks to improve website accessibility

[speth]

I spent some time with Lighthouse, which provides automated checks for a number of accessibility issues, and these were the ones I was able to fix by adjusting our CSS rules and HTML templates.

Changes proposed in this pull request

• Adjust some colors to increase contrast between text and background
• Fix links in Doxygen lists so that they are visibly highlighted when navigating with the "tab" key
• Add the lang="en" page to Doxygen-generated pages

If applicable, fill in the issue number this pull request is fixing

Partially addresses Cantera/enhancements#41

If applicable, provide an example illustrating new features this pull request is introducing

Checklist

• [x] The pull request includes a clear description of this code change
• [x] Commit messages have short titles and reference relevant issues
• [x] Build passes (scons build & scons test) and unit tests address code coverage
• [x] Style & formatting of contributed code follows contributing guidelines
• [x] The pull request is ready for review

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.13%. Comparing base (a1ee588) to head (c2e4fdf). Report is 31 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1753 +/- ## ========================================== + Coverage 73.04% 73.13% +0.09% ========================================== Files 381 381 Lines 54164 54134 -30 Branches 9240 9222 -18 ========================================== + Hits 39562 39590 +28 + Misses 11633 11587 -46 + Partials 2969 2957 -12 ```

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