Minor: Capitalization: learn (Grid -> grid, Flexbox -> flexbox)

[estelle]

Learn section only:

• change title case to sentence case for articles.
• lowercase flexbox and grid in middle of sentences
• remove "guide" in guide links

glossary section is at #34301, with more to come.

part of CSS baseline / interop openwebdocs/project#189 - subcomponent openwebdocs/project#202

[github-actions]

Preview URLs (14 pages)

• /en-US/docs/Learn/CSS/Building_blocks/Organizing
• /en-US/docs/Learn/CSS/CSS_layout/Flexbox
• /en-US/docs/Learn/CSS/CSS_layout/Fundamental_Layout_Comprehension
• /en-US/docs/Learn/CSS/CSS_layout/Grids
• /en-US/docs/Learn/CSS/CSS_layout/Introduction
• /en-US/docs/Learn/CSS/CSS_layout/Legacy_Layout_Methods
• /en-US/docs/Learn/CSS/CSS_layout/Responsive_Design
• /en-US/docs/Learn/CSS/CSS_layout
• /en-US/docs/Learn/CSS/Howto/Center_an_item
• /en-US/docs/Learn/CSS/Howto
• /en-US/docs/Learn/Common_questions/Tools_and_setup/What_are_browser_developer_tools
• /en-US/docs/Learn/Forms/Styling_web_forms
• /en-US/docs/Learn/HTML/Howto/Author_fast-loading_HTML_pages
• /en-US/docs/Learn/JavaScript/Client-side_web_APIs/Video_and_audio_APIs

External URLs (8)

URL: /en-US/docs/Learn/HTML/Howto/Author_fast-loading_HTML_pages
Title: Tips for authoring fast-loading HTML pages

• https://www.w3.org/TR/css-flexbox-1/ (1 time) (Note! This may be a new URL 👀)
• https://www.w3.org/TR/css-grid-1/ (1 time) (Note! This may be a new URL 👀)

---

URL: /en-US/docs/Learn/CSS/CSS_layout/Legacy_Layout_Methods
Title: Legacy layout methods

• https://get.foundation/ (1 time) (Note! This may be a new URL 👀)
• https://getbootstrap.com/ (1 time) (Note! This may be a new URL 👀)

---

URL: /en-US/docs/Learn/CSS/CSS_layout/Responsive_Design
Title: Responsive design

• https://css-tricks.com/a-complete-guide-to-css-media-queries/ (1 time) (Note! This may be a new URL 👀)

---

URL: /en-US/docs/Learn/CSS/CSS_layout/Grids
Title: Grids

• https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/examine_grid_layouts/index.html (2 times) (Note! This may be a new URL 👀)

---

URL: /en-US/docs/Learn/CSS/CSS_layout/Flexbox
Title: Flexbox

• https://css-tricks.com/snippets/css/a-guide-to-flexbox/ (1 time) (Note! This may be a new URL 👀)
• https://flexboxfroggy.com/ (1 time) (Note! This may be a new URL 👀)