[v16] Fix docs markup errors

[iAdramelk]

Update of #42512 based on v16 branch. These changes should also be backported to master.

Some fixes for the docs to fix validation errors and warnings in Docusaurus:

• Remove duplicate redirects.
• Remove redirects form existing pages.
• Update redirects leading to non-existing pages.
• Fix damaged svg image (can't read width and height).
• Add .mdx extension to links to make possible to validate links.
• Fixed typo in docs/pages/access-controls/access-request-plugins/servicenow.mdx inline code.
• Fixed typo in docs/pages/reference/terraform-provider.mdx that was causing mdx failing. (But this is an auto-generated file, so it's better to fix it in the original location too).
• Fixed dbname key for tctl-auth-sign-3-files.mdx includes to make them work.

Didn't fix, but should be fixed separately:

• Link to includes/dynamodb-iam-policy.mdx in CHANGELOG.md – files in includes folder does not build to full pages in docs.
• Three images are used in docs, but are missing in the filesystem:
  • /docs/img/database-access/guides/cloudsql/instance-id@2x.png
  • /docs/img/database-access/guides/cloudsql/instance-public-ip@2x.png
  • /docs/img/desktop-access/install-active-directory.png

[ptgott]

Going to create a buddy PR for this

[iAdramelk]

Hey. What are our next steps on these PRs? I see approvals, but some of the CI task are still falling and also @ptgott wrote that he wants to create buddy PRs for them. What needs to do what next to make these changes merged

[rosstimothy]

External contributions are unable to be accepted directly because CI will not run. @ptgott opened a "buddy" PR which will be merged to master, and then backported to branch/v16, branch/v15, and branch/v14. You will retain authorship of your commits. I'll close this for now to eliminate confusion.

[iAdramelk]

@rosstimothy got it!

Can we do the same with #42510 and #42509 then?

[rosstimothy]

@iAdramelk feel free to close those PRs. @ptgott will take care of backporting the changes to release branches after they land in master.

[iAdramelk]

@rosstimothy I will close them for now. JFY, they have different set of changes. Some changes which were needed for the older versions are not needed in master and vice versa.