Serve tala.run and the documentation of talac and Tazel

The language now lives at tala.run. One frontend build serves three
sites and picks its content from the host name:

- tala.run: the home page, /docs/overview, /docs/getting-started and a
  new /docs/syntax, a first look at programs, comments, declarations,
  operators and evaluation by need, with every sample taken from the
  public specification.
- talac.tala.run: /docs/command-line and /docs/outputs, the compiler's
  options, the entry rule, warnings, --build and --emit, and the record
  of a check.
- tazel.tala.run: /docs/commands and /docs/project-file, the commands of
  the build system, target selection, workspaces and the project DSL.

The new pages are written in all four locales from one content model;
a test keeps code samples and option names identical across them. The
header links the three sites and forum.tala.run. On a development host
?site=talac or ?site=tazel selects a site.

Corrections to the existing pages:

- Getting started used `go run scripts/develop.go`, `talac check -File`,
  `talac build -File` and `-Help`; it now uses `go run ./helpers/cmd/...`,
  `--emit=metadata`, `--entry=` and `--help`.
- The overview no longer names the retired .vxse extension, and the
  project DSL is evaluated by Tazel.
- Canonical URLs, Open Graph data, robots.txt and the sitemap name
  tala.run; talac and tazel have their own robots and sitemap files,
  served per host by Nginx. The notice about the move is gone: the
  former domain redirects instead.
- The leopard mark files are called tala-mark.svg and tala.svg.

Nginx: ops/nginx/tala.conf serves the three hosts, redirects www.tala.run
to tala.run and every path of xsharp-lang.xyz to the same path of
tala.run, and keeps viget.xsharp-lang.xyz pointing at
viget.progmasoft.com; tala.run has no ViGet host of its own.
ops/nginx/forum.conf serves the informational page at forum.tala.run and
redirects the former forum address to it.

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