Make `vector generate` output documentation

Right now vector generate outputs quite minimal configurations.

❯ vector generate "stdin"
data_dir = '/var/lib/vector/'
dns_servers = []
[sources.source0]
max_length = 102400
type = 'stdin'

This issue proposes the output be something like the following:

#                                    __   __  __
#                                    \ \ / / / /
#                                     \ V / / /
#                                      \_/  \/
#
#                                    V E C T O R
#                                   Configuration
#
# ------------------------------------------------------------------------------
# Website: https://vector.dev
# Docs: https://vector.dev/docs/
# ------------------------------------------------------------------------------

# The location of data storage
data_dir = '/var/lib/vector/'
# Custom DNS servers to use (if any)
dns_servers = []

[sources.source0]
# https://vector.dev/docs/reference/sources/stdin/
type = 'stdin'
# The maxiumum bytes size of a message before it is discarded.
max_length = 102400

We may want to opt for a smaller logo, or none at all. We also may want to output commented versions of fields that are using defaults.

Start by locating the implementation and tests for the vector generate command, then run the example command to inspect its current output. Done should produce configuration with explanatory comments, source documentation links, and possibly commented defaults, while resolving the open question about the logo size or omission.