* Fix all issues reported by remark

* Move remark dependencies from the workflow definition into package.json

This enables the transitive dependencies to be pinned, just like for all
other packages.

* Add additional remark plugins

These are needed to correctly parse certain constructs in Markdown files
(such as tables and YAML frontmatter), and without them, remark produces
invalid warnings on some files.

* Update the remark-lint preset versions

The previous versions reference the old version of the
`remark-lint-table-cell-padding` plugin, which doesn't work correctly with
the current version of `remark-parse` (and thus produces spurious warnings).

* GitHub Actions: run remark on all Markdown files, not just changed ones

This way, if a PR updates the remark configuration and that causes new errors
to appear, those errors will show up in that PR, instead of the (unrelated) PR
that next updates the affected files.

There is no runtime cost to this, because remark takes approximately 1 second
to check all files.