Add Markdown link checking

[beanscg]

Describe your changes

Add the official lychee pre-commit hook for Markdown links with fragment checking enabled.

Add a PR-template reminder to update contributor workflow docs when those workflows change.

Fix two existing local Markdown links so the new hook passes on the current tree.

Issue Link

Closes #654.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (Addition or improvements to documentation)

Checklist before requesting a review

• My branch is up-to-date with the target branch - if not update your fork with the changes from the target branch (use pull with --rebase option if possible).
• I have performed a self-review of my code
• For any new/modified functions/classes I have added docstrings that clearly describe its purpose, expected inputs and returned values
• I have placed in-line comments to clarify the intent of any hard-to-understand passages of my code
• I have updated the README to cover introduced code changes
• If this PR changes contributor workflow, I've updated CONTRIBUTING.md / AGENTS.md
• I have added tests that prove my fix is effective or that my feature works
• I have given the PR a name that clearly describes the change, written in imperative form (context).
• I have requested a reviewer and an assignee (assignee is responsible for merging). This applies only if you have write access to the repo, otherwise feel free to tag a maintainer to add a reviewer and assignee.

Checklist for reviewers

Each PR comes with its own improvements and flaws. The reviewer should check the following:

• the code is readable
• the code is well tested
• the code is documented (including return types and parameters)
• the code is easy to maintain

Author checklist after completed review

• I have added a line to the CHANGELOG describing this change, in a section
  reflecting type of change (add section where missing):
  • added: when you have added new functionality
  • changed: when default behaviour of the code has been changed
  • fixes: when your contribution fixes a bug
  • maintenance: when your contribution is relates to repo maintenance, e.g. CI/CD or documentation

Checklist for assignee

• PR is up to date with the base branch
• the tests pass
• (if the PR is not just maintenance/bugfix) the PR is assigned to the next milestone. If it is not, propose it for a future milestone.
• author has added an entry to the changelog (and designated the change as added, changed, fixed or maintenance)
• Once the PR is ready to be merged, squash commits and merge the PR.

Verification

• uvx pre-commit run check-yaml --files .pre-commit-config.yaml
• uvx pre-commit run lychee --all-files
• uvx pre-commit run trailing-whitespace --files .pre-commit-config.yaml .github/pull_request_template.md README.md CHANGELOG.md lychee.toml
• uvx pre-commit run end-of-file-fixer --files .pre-commit-config.yaml .github/pull_request_template.md README.md CHANGELOG.md lychee.toml
• uvx pre-commit run codespell --files .pre-commit-config.yaml .github/pull_request_template.md README.md CHANGELOG.md lychee.toml
• git diff --check

[sadamov]

@beanscg could you comment in #65 so that I can assign this issue to you?

[joeloskarsson]

^ *#654