Enhance content on installation access tokens for GitHub Apps

[mcummings128]

Expanded the explanation of installation access tokens, emphasizing their short-lived nature and scope inheritance from the associated GitHub App.

Why:

Closes: TBD

What's being changed (if available, include any code snippets, screenshots, or gifs):

See above. The section "About installation access tokens" should explain what these tokens are and characteristics regarding them. The way the page is currently set up does not give much information as to what installation access tokens actually are--it just says they're needed to auth a Github App. While that's true, this doesn't explain their benefits. I'd argue that their benefits could even be further embellished upon than what I wrote.

Most of the information in the added text was pulled from the page Authenticating as a GitHub App installation

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
apps/creating-github-apps/authenticating-with-a-github-app/generating-an-installation-access-token-for-a-github-app.md	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[erik225-lgtm]

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