Expand plain language documentation with best practices

[huda-code]

Added sections on the importance of plain language, best practices, and federal resources to enhance understanding and usability.

module-name: One line description of your change (less than 72 characters)

Problem

Explain the context and why you're making that change. What is the problem
you're trying to solve? In some cases there is not a problem and this can be
thought of being the motivation for your change.

Solution

Describe the modifications you've done.

Result

What will change as a result of your pull request? Note that sometimes this
section is unnecessary because it is self-explanatory based on the solution.

Some important notes regarding the summary line:

• Describe what was done; not the result
• Use the active voice
• Use the present tense
• Capitalize properly
• Do not end in a period — this is a title/subject
• Prefix the subject with its scope

Test Plan

(Write your test plan here. If you changed any code, please provide us with
clear instructions on how you verified your changes work.)

[natalialuzuriaga]

Hi @huda-code,

Thank you so much for this valuable contribution to our ospo-guide. We've been meaning to update our plain language guide so this work is so appreciated.

A couple of things before we ship this guide:

1. For documentation, please fill out the PR template with information about the Problem, Solution, and Result. Note that this PR address this issue: #32
2. I flagged changes in markdown file to be addressed

Thank you!

[natalialuzuriaga]

Currently, these websites are plain text. Could you add them as links to these resources so it would be helpful for users to check out

[natalialuzuriaga]

Please make "digital.gov" a link to the digital.gov plain language guide

[natalialuzuriaga]

Lines 1-15 are deleted but this is front matter needed for the website so it displays successfully at https://dsacms.github.io/ospo-guide/growing/plain-language/. Please return this!