Document dev database seed rake command

What does this MR do?

After running bundle exec rake db:reset RAILS_ENV=development, trying to run bundle exec rake db:setup RAILS_ENV=development returns:

Database 'gitlabhq_development' already exists
Database 'gitlabhq_test' already exists
psql:/home/lee/gitlab-development-kit/gitlab/db/structure.sql:1: ERROR:  schema "gitlab_partitions_dynamic" already exists
rake aborted!
failed to execute:
psql --set ON_ERROR_STOP=1 --quiet --no-psqlrc --file /home/lee/gitlab-development-kit/gitlab/db/structure.sql --single-transaction gitlabhq_development

Please check the output above for any errors and make sure that `psql` is installed in your PATH and has proper permissions.

/home/lee/gitlab-development-kit/gitlab/lib/gitlab/database/postgresql_database_tasks/load_schema_versions_mixin.rb:10:in `structure_load'
/home/lee/.asdf/installs/ruby/2.7.2/bin/bundle:23:in `load'
/home/lee/.asdf/installs/ruby/2.7.2/bin/bundle:23:in `<main>'
Tasks: TOP => db:setup => db:schema:load
(See full trace by running task with --trace) 

This MR adds the command which successfully reseeds the db bundle exec rake dev:setup

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team