docs: Add initial docs

[bradenhilton]

Same order as the FuncMap for now. Splitting or re-ordering can be done later.

I feel like some of them aren't quite right...

[twpayne]

This is great, thank you!

[bradenhilton]

I'll merge this, but I'm still not quite sure how to achieve what you outlined in #1 (comment), both how to set up embed, and how to consume it within chezmoi, having never really done anything like this before.

[twpayne]

how to achieve what you outlined in #1 (comment)

chezmoi uses this for its documentation and examples. The rough outline is:

1. The markdown documentation is included in chezmoi's binary using the Go standard library's embed package.
2. chezmoi includes a markdown parser and renderer and uses this to extract relevant bits of documentation.

The relevant chezmoi code is:

1. https://github.com/twpayne/chezmoi/blob/4504bc3b4fd221d4a20e29a2b95ee9cffebadb10/assets/chezmoi.io/docs/reference/commands/commands.go makes all the command *.md files available in a io/fs.FS. This is in the Go package github.com/twpayne/chezmoi/v2/assets/chezmoi.io/docs/references/commands.
2. This package is imported and, on startup, the command *.md files are parsed and the help text extracted: https://github.com/twpayne/chezmoi/blob/4504bc3b4fd221d4a20e29a2b95ee9cffebadb10/internal/cmd/cmd.go#L80-L91. The actual parsing happens here: https://github.com/twpayne/chezmoi/blob/4504bc3b4fd221d4a20e29a2b95ee9cffebadb10/internal/cmd/cmd.go#L152. It's a crude regexp-based state machine that extracts sections of the markdown files based on section titles.
3. The extracted markdown sections are rendered as plain text and stored in strings: https://github.com/twpayne/chezmoi/blob/4504bc3b4fd221d4a20e29a2b95ee9cffebadb10/internal/cmd/cmd.go#L199-L210.
4. Later, the strings are used in chezmoi's help output.

For template funcs, what's needed it for the docs subdirectory to contain a .go file that:

1. Embeds templatefuncs.md.
2. Includes an init() function that parses templatefuncs.md and populates an exported map[string]Reference, something like:

package docs

import (
    _ "embed"
)

//go:embed templatefuncs.md
var templateFuncsStr string

type Reference struct {
    Title   string
    Body    string
    Example string
}

var References map[string]Reference

func init() {
    // Parse templateFuncStr and store the results in References.
}

References then contains something like:

References["contains"] = Reference{
    Title: "`contains` *substring* *string*",
    Body:  "`contains` returns whether *substring* is in *string*.".
    Example: "```text\n" +
             "{{ "abc" | contains "ab" }}\n" +
             "\n" +
             "true\n" +
             "```\n",
}

Then, finally, a tool in chezmoi's build system imports github.com/chezmoi/templatefuncs/docs and generates Markdown files for chezmoi's website from the contents of the References variable.