Fix markdownlint warnings, add documentation on bugtool plugins

[bernhardkaindl]

My brief summary

Improve the legibility of the Markdown documentation files for xen-bugtool by addressing the markdownlint warnings, which are intended to make Markdown files easier to read.

More detailed summary based on review by Sourcery AI

Added documentation enhancements

1. Clean up markdown formatting in conftest, pre-commit, testing, README, release, coverage, and install docs
2. Add documentation describing xen-bugtool’s plugin XML structure, workflow, and best practice: doc/plugins.md

Added Markdown linting checks and configuration

• Add a markdownlint hook and configuration to .pre-commit-config.yaml
• Add a markdownlint configuration file .markdownlint.yaml to tune specific linting rules

[Copilot]

Pull Request Overview

This PR fixes markdownlint warnings across documentation files and establishes markdownlint as a pre-commit hook to maintain consistent markdown formatting. It also adds comprehensive documentation for xen-bugtool's plugin system.

• Fix markdownlint formatting issues across multiple documentation files
• Add markdownlint configuration and pre-commit hook for automated checking
• Create detailed plugin documentation explaining XML structure and workflow

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/unit/conftest-README.md	Fix markdown formatting issues and improve documentation structure
doc/testing.md	Fix header formatting and typos, improve list structure
doc/release.md	Add missing blank lines around code blocks and improve formatting
doc/pre-commit.md	Fix header levels, improve link formatting, and fix typos
doc/plugins.md	New documentation explaining xen-bugtool plugin XML structure and workflow
doc/coverage.md	Fix header level from ## to #
README.md	Improve organization and add missing blank lines
README-python-install.md	Fix header level and spelling error
README-Windows-WSL2.md	Fix header level formatting
.pre-commit-config.yaml	Add markdownlint hook configuration
.markdownlint.yaml	Add markdownlint configuration with project-specific rules

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