Convert hotdoc metadata to Hugo frontmatter

Convert the existing metadata stored at the front of each file to
frontmatter formatted for Hugo:

 - Remove authors as we don't have an equivalent for that in Hugo
 - Add in fields that the existing metadata doesn't have
 - Include aliases to allow us to point to the new pages from a
   redirect on designs.apertis.org
 - Ensure we have a title (based on filename if one not present)

All frontmatter created in toml format to stay consistent with existing
documents.

Converted using the following python script:
```

import os
import re
import sys
import toml
import yaml

for filename in os.listdir(sys.argv[1]):
    print("%s: " % filename)
    with open(filename, 'r+') as file:
        contents = file.read()
        if contents[0:3] == "+++":
            # We have toml
            data = toml.loads(contents.split("+++")[1])
            doc = "+++".join(contents.split("+++")[2:])
        elif contents[0:3] == "---":
            # We have yaml
            data = yaml.load(contents.split("---")[1])
            doc = "---".join(contents.split("---")[2:])
        else:
            # No frontmatter
            data = {}
            doc = contents

        if not "title" in data.keys():
            data["title"] = filename.split(".")[0].replace('_', ' ').capitalize()

        data["weight"] = 100
        data["aliases"] = ["/old-designs/latest/%s" % filename.replace(".md", ".html")]

        data["outputs"] = ["html", "pdf-in"]

        if "authors" in data.keys():
            del data["authors"]

        file.seek(0)
        file.truncate()

        file.write("+++\n")
        file.write(toml.dumps(data))
        file.write("+++")
        file.write(doc)
```

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>