Document different ways to consume inputs

Also laying out how e.g. process substitution is handled, and why one
might need to override the file name for that.

site/content/ytt/docs/develop/inputs.md

@@ -0,0 +1,93 @@
+---
+title: Inputs
+---
+
+ytt supports different input sources:
+
+- Files & Directories
+  - Those are provided via the `-f`/`--file` flag
+
+  - ytt uses the file's name for its internal representation
+
+    If you have a tree like
+    ```terminal
+    $ tree .
+    .
+    ├── dir1
+    │   └── some.yaml
+    └── dir2
+        └── sub
+            ├── another.yaml
+            └── someother.yaml
+
+    4 directories, 3 files
+    ```
+    and you call `ytt --file dir1/some.yaml --file dir2/ ...` then ytt loads
+    - `dir1/some.yaml` as `some.yaml`
+    - `dir2/sub/another.yaml` as `sub/another.yaml`
+    - `dir2/sub/someother.yaml` as `sub/someother.yaml`
+
+  - ytt uses a file's extension to determine its type, e.g. a extension like
+    `yaml` flags that file as "yaml-template"; you can read more about that in
+    [File Marks](file-marks/)
+
+  - You can change a file's name, location, and also "type" by explicitly
+    setting the file's name to be used by ytt, e.g. `ytt --file
+    a/different/file.foo=dir1/some.yaml`, which would mean that ytt
+
+    - loads that file as `a/different/file.foo`
+    - would not consider it as "yaml-template"/"yaml-plain", but as "data", because of its extension
+
+    Note: this only works for files, not for directories
+
+  - Explicitly setting file's names can be especially useful when consuming
+    files where you have no control over their name, like process substitutions:
+
+    Running `ytt --file <(echo 'some: yaml')` (on Linux) would have the shell
+    produce a file like `/dev/fd/63` and pass that on to ytt. This file, based
+    on it's name "63", would not be considered yaml and thus interpreted as
+    "data". To change that, you need to run `ytt --file subst.yaml=<(echo 'some:
+    yaml')` to have ytt treat it as yaml.
+
+  - ytt can also consume stdin by using `-`, like: `ytt --file -`
+
+    Note: When using `-`, ytt automatically treats data on stdin as yaml, as it
+    will use stdin as `stdin.yaml`, thus having an extension which flags it as
+    "yaml-template". If you use some other means to consume stdin, e.g. `ytt
+    --file /dev/stdin`, this does not happen and ytt treats stdin as a file
+    `stdin` and thus as "data", because it has no extension marking it
+    differently. You can still set a different file name explicitly, e.g. with
+    `ytt --file my-stdin.yaml=/dev/stdin`.
+
+  - ytt can also consume files via http/s, e.g. `ytt --file
+    https://raw.githubusercontent.com/carvel-dev/ytt/develop/.golangci.yml`
+
+  - ytt can also consume symlinks, however if a symlink's target is not a file
+    you have already included into the set of files ytt should consider
+    (`--file ...`), ytt will not allow that and print an error. You can
+    explicitly allow additional symlink targets via the
+    `--allow-symlink-destination ...` flag.
+
+  - To debug / inspect which files ytt considers and how it handles those, the
+    flags `--files-inspect` & `--debug` can be helpful
+
+
+- Data Values & Data Values Schemas
+
+  You can read about how to define data-values schemas and how to consume and
+  set Data Values and Schemas here:
+
+  - [Write Schema](how-to-write-schema/)
+  - [Data Values Schema](lang-ref-ytt-schema/)
+  - [Use Data Values](how-to-use-data-values/)
+  - [Data Values](ytt-data-values/)
+
+  Generally you can provide Data Values
+  - as strings
+    - on the command line via the flags `--data-value`/`-v`
+    - from the environment via the flag `--data-values-env`
+    - from files via the flag `--data-value-file`
+  - as structured data / yaml
+    - on the command line via the flag `--data-value-yaml`
+    - from the environment via the flag `--data-values-env-yaml`
+    - from files via the flag `--data-values-file`

site/data/ytt/docs/ytt-develop-toc.yml

@@ -85,6 +85,8 @@ toc:
         url: /lang-ref-load
   - title: CLI configuration
     subfolderitems:
+      - page: Inputs
+        url: /inputs
       - page: Outputs
         url: /outputs
       - page: File Marks