docs: fix typos, improve clarity and structure across plugin guides

[Tomasz Turkowski (tturkowski)]

Summary

• Fixed typos and grammatical errors across 17 plugin documentation files
• Improved structural consistency: removed redundant ## Overview headings, corrected heading levels (##/###/####), moved orphaned paragraphs to appropriate sections
• Replaced informal language ("so no worries!", "And that's it", "take a look at", "we recommend") with neutral documentation tone
• Fixed code snippet issues: corrected // comments inside Twig/XML blocks, fixed malformed JSON indentation, removed invalid JS syntax ({};)
• Standardized internal link paths (removed unnecessary ../plugins/ prefix)
• Added missing code examples to ### Input fields and ### Label, placeholder and help text in add-plugin-configuration.md
• Added minimal CLI command class example to add-custom-commands.md
• Fixed factually incorrect statements in add-scheduled-task.md (run method description did not match the code)
• Fixed two typos in mcp-server.md (passes read → reads, passes remap → remaps)

Related links

None.

Checklist

• I reviewed affected links, code samples, and cross-references, including PageRef references where relevant.
• I added or updated redirects in .gitbook.yaml if pages were moved, renamed, or deleted.
• I updated .wordlist.txt (and sorted it) if spellcheck flags new legitimate terms.
• Any required dependent changes in downstream modules have already been merged and published.
• This pull request is ready for review.

Notes

This PR covers:

• guides/plugins/plugins/
• guides/plugins/plugins/plugin-fundamentals/
• guides/plugins/plugins/database
• guides/plugins/plugins/services

There is more to work on, but I want to avoid creating a large PR, therefore here's part 1.