docs: 'Creating Custom Alphabets' guide teaches the v5 format — needs a v6 rewrite #41

Problem

docs/languages/creating-alphabets.md documents the v5 alphabet format:

• <group label="..."> + <char d="a" /> — v6 uses <group name="..."> + <node label="a"><textCharAction/></node> (the v5 <s d="" t=""> element still parses but is legacy)
• No mention of the v6 root attributes that matter: orientation (LR/RL/TB/BT), trainingFilename, colorsName, conversionMode
• No mention of the naming convention (alphabet.<name>.xml) or where the file goes on each v6 frontend (GTK: ~/.local/share/dasher/alphabets/ — see the updated downloads page)
• No pointer to Data/alphabets/ConvertAlphabet.py for v5→v6 conversion
• New for v6: adding an alphabet should regenerate alphabet_index.json (Scripts/generate-alphabet-index.py in DasherCore) so the catalogue page and metadata stay current — the guide should mention it

What good looks like

1. A minimal working v6 example (comment the structure)
2. Attributes reference (name, orientation, trainingFilename, colorsName, conversionMode)
3. Groups and nodes; textCharAction; control actions
4. Training text: how to provide trainingFilename, size guidance (300K+), where it goes
5. Testing it: drop in the user dir, restart, select in Settings
6. Contributing it upstream: file layout, regeneration of the index

Context: with v6 shipping all 474 alphabets built in (website PR #40), this guide's audience is people adding genuinely new languages or custom symbol sets — worth making correct.