Evaluate contained overlay directives when target fragment is missing from "left"

**As a** Configuration Author
**I want** the overlay directives I include within an optional fragment (i.e. `missing_ok=True`) to be evaluated (when the parent of the fragment is not present in the target on the "left", the directives are not being evalutated.
**so that** overlays behave the way I expect, rather than insert a chunk of plain YAML.

---

## 🟢 Main Flow

**Given** an overlay that contains a fragment that optionally matches
**And** that fragment _itself_ contains overlay directives

`config1.yml`
```yaml
---
foo: 42
```

`config2.yml`
```yaml
---
foo: 42
bar:
qux: deadbeef
quuz: prefix
list:
- zeroth
```

`overlay.yml`
```yaml
#@ load("@ytt:overlay", "overlay")

#@overlay/match by=overlay.all, expects="1+"
---
#@overlay/match missing_ok=True
bar:
#@overlay/match missing_ok=True
#@overlay/remove
qux: toasty
#@overlay/match missing_ok=True
baz: added from overlay
#@overlay/match when=1
#@overlay/replace via=lambda left,right: left + "-" + right
quuz: suffix

#@overlay/match missing_ok=True
list:
- first
#@overlay/insert after=True
- second
```

**When** I run this through ytt
```console
$ ytt -f config1.yml -f config2.yml -f overlay.yml
```

**Then** I should see that those overlay directives (e.g. the `@overlay/remove` on `ree:`) contained within the inserted parent (i.e. `bar:`) are executed:

`config1.yml`
```yaml
---
foo: 42
bar:
baz: added from overlay
list:
- first
- second
```

`config2.yml`
```yaml
---
foo: 42
bar:
baz: added from overlay
quuz: prefix-suffix
list:
- zeroth
- first
- second
```

... instead of https://carvel.dev/ytt/#gist:https://gist.github.com/pivotaljohn/281e6c3d1d77c7e2b1e4763a0f400add

Explicitly:
- `bar.qux:` does not appear in either result given that it's `@overlay/remove`d
- `bar.quuz:`
- appears only in `config2.yml` given that it's `@overlay/match when=1`
- that the value is calculated based on the expression given to `@overlay/replace`
- `list:`
- appears in both `config1.yml` and `config2.yml`
- "second" is the 3rd item in the list in `config2.yml` given that it's `@overlay/insert`ed there.

## 📖 Documentation

**Given** I am looking to understand how overlay directives behave when their parent matched via `missing_ok=True` (or `expects="0+"` or similar)
**When** I skim through the docs
**Then** I see this behavior explicitly described

---
Note:
- for directives that present the "left side" to the author (i.e. `@overlay/match`, `@overlay/replace`, `@overlay/assert`), the value for `left` should be `None`.

---
To Do:
- [ ] when complete, resolve/close #308.

Start by running the ytt command with the config1.yml, config2.yml, and overlay.yml examples from the issue, then inspect the overlay directive evaluation path. Done means contained directives run when a missing_ok=True parent is inserted, producing the shown YAML results, handling left as None where specified, and documenting the behavior; resolve #308 when complete.

Written by the indexing model from the issue text.