Docs v0: improve quickstart wording

[rahulshendre]

What this PR does:

Why we need it:
minor clean up

Does this PR introduce a user-facing change?:

• How are users affected by this change: docs wording only
• Is this breaking change: no
• How to migrate (if breaking change): n/a

[armistcxy]

Overall LGTM, but can you clarify whether this part will work if we switch from /docs/ to ../: https://github.com/pipe-cd/pipecd/pull/6982/changes#diff-f55bb21231f50db5ceb096ad64110cadd5613eb9f20061e828081ee10b72739cR77

[armistcxy]

Looks good overall

[rahulshendre]

Overall LGTM, but can you clarify whether this part will work if we switch from /docs/ to ../: https://github.com/pipe-cd/pipecd/pull/6982/changes#diff-f55bb21231f50db5ceb096ad64110cadd5613eb9f20061e828081ee10b72739cR77

I checked this on the local build, the quickstart page sits inside the version folder (/docs-v0.57.x/quickstart/), so ../user-guide/ takes you to the user guide of the same version.
The /docs/ links work on the live site because it redirects to the latest version, but on a local build they don't go anywhere, so relative felt like the safer choice here.

also once 0.58 ships, /docs/ would send someone reading the 0.57 quickstart into the 0.58 docs, ../ keeps them on the same version.

[mohammedfirdouss]

@rahulshendre take a look at the failed tests

[rahulshendre]

@mohammedfirdouss, resolved the conflict with #6791, it touched the same Control Plane line in the docs-de, all green now, the lint fail earlier is also cleared on the rerun.

[github-actions]

Thank you for contributing to PipeCD, @rahulshendre! The changes in this pull request will be part of the upcoming release!