Content-preserving release content TOML

## Problem

There are a few issues in how we generate the initial templates for our release notes and migration guides. Namely, the TOML manifests (`_guides.toml` and `_release-notes.toml`) do not preserve user edits. This is an issue because PRs are frequently merged, cut, or edited in ways that our current tooling just skips over.

For an example, in #1340 some migration guides were deleted. In #1366 when I re-ran the tool to synchronize new changes the deleted guides were re-generated, so I had to manually delete them. This is especially challenging due to our release candidate process, which encourages frequent synchronizations of migration guides.

## Solution

Create a data-model for `_guides.toml` and `_release-notes.toml` within `generate-release` using `serde`, so we can serialize and deserialize the data. (For context, we use [string templates](https://github.com/bevyengine/bevy-website/blob/8699f1b4379eb142efa2489ddff959ed874ef570/generate-release/src/migration_guides.rs#L123-L127) right now.) With this, we can use [`toml_edit`](https://docs.rs/toml_edit/latest/toml_edit/) to edit the existing manifest TOMLs without overwriting them.

With `toml_edit`, we can load any existing TOML file and edit it as we wish, while still preserving order and comments. The new system will try to load an existing `_guides.toml` or `_release-notes.toml` if it exists and edit it instead.

In the following sections I'll lay out the strategies to handling different kinds of changes. I'll use the bird emoji (🐦) to represent migration guides and the notebook emoji (📓) to represent release notes.

### Merged PRs (🐦📓)

At the top of each TOML file will be the `[manifest]` table with the `exclude` property, which lists the PR numbers that are to be skipped.

```toml
[manifest]
exclude = ["12038", "12512"]

[[guides]]
# ...
```

In order to merge multiple PRs together, you must add all but one of them to the `exclude` list. The one that is not excluded will contain the merged content. (In the future, the `[[guides]].url` value could be updated to accept a list of URLs instead of only one.)

### Reordered PRs (🐦📓)

With `toml_edit`, order is automatically preserved. Nothing else needs to be done in this circumstance.

### Changed titles (🐦📓)

`generate-release` will never try to overwrite existing titles, even if they differ from the PR title on Github. You may edit this as you please.

### Changed URL (🐦📓)

This should never be done, as this is how `generate-release` identifies which PR corresponds to which entry.

### Changed filename (🐦📓)

This should also never be done. The filenames are not public-facing, and thus not worth the time renaming. `generate-release` probably will not overwrite this value for efficiency's sake, but you are still discouraged from changing it.

### Changed area (🐦)

This should not be changed in `_guides.toml`, but instead the tags should be modified on the original Github PR. The tool should then automatically pick up new tags and overwrite existing `area`s.

This may be an issue if you merge PRs with different tags. In this case, I'd recommend just leaving them separate. If they affect different areas of Bevy, should they really be combined together?

### Changed author (📓)

Like titles, this field may be modified at will without being overwritten. The author field is calculated by finding the authors of all commits in that PR, even really small changes like typo suggestions and whitespace. In this case, it is better to remove that person's name from the author field so that others do not approach them with questions about the PR that they might not be able to answer.

### Cut PRs (🐦📓)

In most cases you should remove the `C-Breaking-Change` or `C-Needs-Release-Notes` label on Github, then re-generate the manifest. In rare scenarios you may employ the `exclude` field instead, but you should add a comment on why you didn't just remove the label.

Start in generate-release, especially migration_guides.rs around the string-template code referenced in the issue, and inspect the _guides.toml and _release-notes.toml manifests. Model both manifests with serde and use toml_edit to update existing files. Done means synchronization preserves order, comments, and user edits while handling merged and excluded PRs as described.

Written by the indexing model from the issue text.