task: add more env var documentation

[jharlow-intel]

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to an issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?
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This PR adds some needed environment variable documentation to the git pages.

[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/2137/index.html

[github-actions]

Array API standard conformance tests for dpctl=0.21.0dev0=py310h93fe807_105 ran successfully.
Passed: 1115
Failed: 6
Skipped: 119

[coveralls]

coverage: 85.902% (+0.005%) from 85.897%
when pulling 5084a64 on task/SAT-7838
into b4e0dca on master.

[github-actions]

Array API standard conformance tests for dpctl=0.21.0dev0=py310h93fe807_105 ran successfully.
Passed: 1115
Failed: 6
Skipped: 119

[github-actions]

Array API standard conformance tests for dpctl=0.21.0dev0=py310h93fe807_107 ran successfully.
Passed: 1114
Failed: 7
Skipped: 119

[github-actions]

Array API standard conformance tests for dpctl=0.21.0dev0=py310h93fe807_105 ran successfully.
Passed: 1114
Failed: 7
Skipped: 119

[github-actions]

Array API standard conformance tests for dpctl=0.21.0dev0=py310h93fe807_105 ran successfully.
Passed: 1114
Failed: 7
Skipped: 119

[github-actions]

Array API standard conformance tests for dpctl=0.21.0dev0=py310h93fe807_105 ran successfully.
Passed: 1114
Failed: 7
Skipped: 119

[github-actions]

Array API standard conformance tests for dpctl=0.21.0dev0=py310h93fe807_105 ran successfully.
Passed: 1115
Failed: 6
Skipped: 119

[jharlow-intel]

@ndgrigorian ready for another review