[vcpkg manifest] Add documentation! (#13488)

* [vcpkg docs] add docs for manifest files

These are just for the maintainer docs,
not user docs.

* [vcpkg] EBNF-ify platform expression parsing

this modifies nothing about what strings are accepted or rejected,
it just moves stuff around.

also adds tests.

* [vcpkg docs] add manifest mode example

* [wip] docs for augustin

also fix tabs

* [vcpkg manifest] switch to using maps for features

* Apply suggestions from code review

* un-experimentize format-manifest

* flesh out the user manifest mode docs

* CRs

* billy CRs

* final personal pass-thru

1 files changed, 3 insertions, 4 deletions

diff --git a/ports/abseil/vcpkg.json b/ports/abseil/vcpkg.json
index 79e6c308b..0fbb7abb9 100644
--- a/ports/abseil/vcpkg.json
+++ b/ports/abseil/vcpkg.json
@@ -9,10 +9,9 @@
     "Abseil is not meant to be a competitor to the standard library; we've just found that many of these utilities serve a purpose within our code base, and we now want to provide those resources to the C++ community as a whole."
   ],
   "homepage": "https://github.com/abseil/abseil-cpp",
-  "features": [
-    {
-      "name": "cxx17",
+  "features": {
+    "cxx17": {
       "description": "Enable compiler C++17."
     }
-  ]
+  }
 }