Eliminate broken link warnings (except those from docs in the main repo)

[mehhhhhhhhhhhhhhh]

Description

Partial fix for the current "broken link" warnings, to unblock https://github.com/opentofu/opentofu.org/issues/85.

For a complete fix, separate PRs will be needed to the four branches of the main https://github.com/opentofu/opentofu repo, as there are also some broken links in each of these. Then the submodule references will need updating, and then all should be well.

Initial draft for that main branch is here: https://github.com/opentofu/opentofu/pull/1884/files

Fair warning in advance: once all the broken links are fixed, Docusaurus reveals further warnings about broken anchors. (The setting defaulting to warn for these is separate from that for the links.) But this is progress, at least.

Motivation and Context

Screenshots (if appropriate):

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [x] My change depends on a change (four changes) in the main opentofu repository.

[mehhhhhhhhhhhhhhh]

@janosdebugs this is ready for review, thanks :bow:

[mehhhhhhhhhhhhhhh]

The commit messages should hopefully explain everything.

The main reason for unexpected changes is that I had to first run prettier on all the existing blog posts, to avoid the included pre-commit hook making the same unrelated changes (mostly line endings, unfortunately) in my commits (or others' future commits) and resulting in confusing diffs.

~The other commit with non-link changes is a version bump to get a fix from Docusaurus that suppresses all the errors about links to /slack/. This is also explained in that commit message.~ Edit: undone after later discussion

[mehhhhhhhhhhhhhhh]

(Sorry, that last force push was just adding signatures :facepalm:)