[docs] Move GOVERNANCE.md and ROADMAP.md files from root

[croraf]

• I have followed (at least) the PR section of the contributing guide.

MOTIVATION
To clean root of the repository a bit, and be more consistent with where other pages from website's "Discover More" sections are placed.

CHANGES

• Moved GOVERNANCE.md and ROADMAP.md files from the root to "/docs/src/pages/discover-more/roadmap/roadmap.md"

• Also added Governance section to the README.md (sentence I took from the governance page).

• For roadmap link in README.md I put lowecase without extension because other links are like that.

• In CONTRIBUTING.md I put a roadmap link to point to website chapter, not to the github file.

[mui-pr-bot]

No bundle size changes comparing 821e4a6...9caa368

Generated by 🚫 dangerJS against 9caa368

[croraf]

@joshwooding @oliviertassinari You can decide if also exclude them from translation or not analogous to 57f818f

[oliviertassinari]

Yes, I would be in favor of it too. The main incentive is that these pages have a low visite rate, reducing the number of pages that can be translated help define what's important and what's not for the translators. The last time I checked, we had x2 more words to translate than Vuetify for fewer components supported. Also, they are probably more useful to English speakers that can interacts with the core team.

[croraf]

These changes from master need to be reaplied

--- a/docs/src/pages/discover-more/roadmap/roadmap.md
+++ b/ROADMAP.md
@@ -54,9 +54,9 @@ We are eager to find new bundle size reduction opportunities. We hope we can lev
 Here are the components we will work on being supported in the Material-UI ecosystem, no matter if they are from @material-ui/core, a third-party project or a paid source:
 
 - 🧪 In the lab, in progress to move into the core
-- 🛠 Work in progress
+- 🛠 Work in progress, will likely land in the lab
 - ⏳ Planning to build
-- ⭐️ At least one part will be paid (non MIT)
+- ⭐️ We will consider making some of the features paid (non MIT)
 
 | Name | Status |
 |------|--------|
@@ -81,7 +81,7 @@ Here are the components we will work on being supported in the Material-UI ecosy
 | Pagination | ⏳ |
 | Scrollspy | ⏳ |
 | TopLayout | ⏳ |
-| Calendar | ⏳⭐️ |
+| Big calendar | ⏳⭐️ |
 | Charts | ⏳⭐️ |
 | Color picker | ⏳⭐️ |
 | Data Grid | ⏳⭐️ |

[oliviertassinari]

@croraf Seems that we are good.