Edit comments in changelog file

[lcawl]

Relates to #2320 (comment)

This PR edits the comments that appear in each generated changelog file, including:

• Grouping them by whether they're required or optional
• Adding the list of valid lifecycle states
• Linking to the list of valid product identifiers

Old comments

##### Automated fields #####

# These fields are likely generated when the changelog is created and unlikely to require edits

# pr: An optional string that contains the pull request number
# issues: An optional array of strings that contain URLs for issues that are relevant to the PR
# type: A required string that contains the type of change
#   It can be one of:
#   - feature
#   - enhancement
#   - bug-fix
#   - known-issue
#   - breaking-change
#   - deprecation
#   - docs
#   - regression
#   - security
#   - other
# subtype: An optional string that applies only to breaking changes
#   It can be one of:
#   - api
#   - behavioral
#   - configuration
#   - dependency
#   - subscription
#   - plugin
#   - security
#   - other
# products: A required array of objects that denote the affected products
#   Each product object contains:
#     - product: A required string with a predefined product ID
#     - target: An optional string with the target version or date
#     - lifecycle: An optional string (preview, beta, ga)
# areas: An optional array of strings that denotes the parts/components/services affected

##### Non-automated fields #####

# These fields might be generated when the changelog is created but are likely to require edits

# title: A required string that is a short, user-facing headline (Max 80 characters)
# description: An optional string that provides additional information (Max 600 characters)
# impact: An optional string that describes how the user's environment is affected
# action: An optional string that describes what users must do to mitigate
# feature-id: An optional string to associate with a unique feature flag
# highlight: An optional boolean for items that should be included in release highlights

New comments

##### Required fields #####

# title:
#   A required string that is a short, user-facing headline.
#   (Max 80 characters)

# type:
#   A required string that contains the type of change
#   It can be one of:
#   - feature
#   - enhancement
#   - bug-fix
#   - known-issue
#   - breaking-change
#   - deprecation
#   - docs
#   - regression
#   - security
#   - other

# products:
#   A required array of objects that denote the affected products
#   Each product object contains:
#
#   - product:
#       A required string with a valid product ID.
#       Valid values are defined in https://github.com/elastic/docs-builder/blob/main/config/products.yml
#
#     target:
#       An optional string with the target version or date.
#
#     lifecycle:
#       An optional string for new features or enhancements that have a specific availability.
#       It can be one of:
#       - preview
#       - beta
#       - ga

##### Optional fields #####

# action:
#   An optional string that describes what users must do to mitigate
#   the impact of a breaking change or known issue.

# areas:
#   An optional array of strings that denotes the parts/components/services
#   of the product that are affected.

# description:
#   An optional string that provides additional information.
#   (Max 600 characters).

# feature-id:
#   An optional string to associate a feature or enhanceent with a
#   unique feature flag.

# highlight:
#   An optional boolean for items that should be included in release
#   highlights or the UI to draw user attention.

# impact:
#   An optional string that describes how the user's environment is
#   affected by a breaking change or known issue.

# issues:
#   An optional array of strings that contain the issues that are
#   relevant to the PR.

# pr:
#   An optional string that contains the pull request number.

# subtype:
#   An optional string that applies only to breaking changes.
#   It can be one of:
#   - api
#   - behavioral
#   - configuration
#   - dependency
#   - subscription
#   - plugin
#   - security
#   - other