install (pyproject): add a main extra to install essential packages:

`all` install everything, `main` only the essential ones.
Also add some comments to explain what the groups are for.

pyproject.toml

@@ -105,14 +105,28 @@ ox = [
     "python-gnupg>=0.5.6",
 ]
 
+# Main features for a good experience
+main = [
+    "libervia-backend[tor]",
+    "libervia-backend[ox]",
+    "libervia-backend[tui]"
+]
+
 # All features
 all = [
+    # Only used by desktop-kivy so far
     "libervia-backend[SVG]",
+    # Needs Qt
     "libervia-backend[GUI]",
+    # Only used for (real) images in terminal.
     "libervia-backend[TUI]",
+    # Following 2 are experimental
     "libervia-backend[remote-control]",
     "libervia-backend[conferences]",
+    # Needs the Tor daémon
     "libervia-backend[tor]",
+    # Used in isolated environment (on system packages, official GPGME Python binding
+    # should be used).
     "libervia-backend[ox]"
 ]