Change logic of input and output targets of post-processing scripts

What does this MR do and why?

Change logic of input and output targets of post-processing scripts. This was mainly done to make minify work again.

• Simplify compress_images.sh to provide only one target.
• Simplify normalize-links.sh to provide output and input targets.
• Simplify minify-assets.sh to provide output and input targets, and run minify only once in the whole site directory.
• Change single.Dockerfile to account for the script changes.

Fixes #1612 (closed).

How to set up and validate locally

Without Docker

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Check out this branch.
3. Compile the site:

   make compile

4. Use the scripts for post-processing:

   scripts/normalize-links.sh public 15.11
   scripts/compress_images.sh public
   mkdir dest
   scripts/minify-assets.sh dest public

5. When minify-assets.sh is run, you should see the following output:

   INFO: Assets minified!
   INFO: Size before minifying: 2.4G
   INFO: Size after minifying: 950M
   INFO: Checking that the number of source and destination files is the same...
   SUCCESS: File count in public and dest: 3005/3005
   INFO: Checking that the number of source and destination directories is the same...
   SUCCESS: Directory count in public and dest: 630/630

Mimicking the Docker build

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Check out this branch.
3. Build the 15.11 image:

   docker build --build-arg NANOC_ENV=production --build-arg VER=15.11 --build-arg SEARCH_BACKEND="lunr" --tag docs:15.11 -f dockerfiles/single.Dockerfile .

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