docs: add missing OpenBao reference to concepts.md

[kangetsu121]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

#1914 added missing documentation for opnebao-csi-providers, but a reference in concepts.md was still missing. This PR adds that missing OpenBao reference and also fixes a few minor Markdown syntax issues.

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):

Special notes for your reviewer:

This is a small documentation-only change.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: kangetsu121 / name: kangetsu121 (87e283b)

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[k8s-ci-robot]

Hi @kangetsu121. Thanks for your PR.

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[aramase]

Please remove all other spurious changes.

[kangetsu121]

@aramase
Thank you for the review and for pointing that out.

I’ve removed the unrelated changes and updated the branch accordingly.
Could you please review it again when you have a chance?

[aramase]

/triage accepted
/ok-to-test

[aramase]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aramase, kangetsu121

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 22.07%. Comparing base (fd0c075) to head (87e283b).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2015   +/-   ##
=======================================
  Coverage   22.07%   22.07%           
=======================================
  Files          57       57           
  Lines        3198     3198           
=======================================
  Hits          706      706           
  Misses       2400     2400           
  Partials       92       92           

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