Proof-read user guide: ironic/introduction

[dtantsur]

• Proper spelling of compound words ("open source" as a noun, "bare-metal" as an adjective, "bare metal" as a noun, etc).
• Mention community as a reason to use Ironic.
• Split text into paragraphs and simplify wording for readability.

[metal3-io-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: furkatgofurov7, tuminoid

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Needs approval from an approver in each of these files: - ~~[docs/user-guide/src/OWNERS](https://github.com/metal3-io/metal3-docs/blob/main/docs/user-guide/src/OWNERS)~~ [furkatgofurov7] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[Rozzii]

/lgtm