docs: document AsciiDoc content conventions (EC-2178)

[BohdanMar]

Summary

• Document the AsciiDoc conventions used in the user guide, with examples for placeholders, page titles, navigation, admonitions, cross-references, and source blocks.
• Note legacy title-casing exceptions and consistent step headings.

Closes #248

Jira: https://redhat.atlassian.net/browse/EC-2178

Validation

• Checked the documented forms against existing .adoc pages.
• git diff --check passed.
• The preview command is blocked by a pre-existing Makefile path mismatch with the current website repository layout.

[coderabbitai]

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