fix: Run on_page_markdown event "first"

[prcr]

Updates on_page_markdown so that it runs before the events from other plugins (as described on mkdocs/mkdocs#2973).

Extra context

After upgrading to mkdocs==1.4.0 and mkdocs-include-markdown-plugin==3.9.0, I started having the following errors when building my site:

$ mkdocs build
INFO     -  Cleaning site directory
INFO     -  Building documentation to directory: /home/prcr/git/docs/site
INFO     -  [macros] - ERROR # _Macro Syntax Error_
            _Line 13 in Markdown file:_ **expected token 'end of statement block', got 'string'**
            ```python
                    include-markdown "../assets/includes/self-hosted-version.txt"
            ```
INFO     -  [macros] - ERROR # _Macro Syntax Error_
            _Line 13 in Markdown file:_ **expected token 'end of statement block', got 'string'**
            ```python
                    include-markdown "../assets/includes/self-hosted-version.txt"
            ```

This is because another plugin that I'm using, mkdocs-macros-plugin==0.7.0, fails to parse the mkdocs-include-markdown-plugin syntax in the Markdown pages.

In practice, the mkdocs-include-markdown-plugin should be one of the very first plugins to run so that the subsequent plugins work on top of the Markdown that already has the included content, as described on the README.

[coveralls]

Pull Request Test Coverage Report for Build 3255930404

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.476%

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Totals
Change from base Build 3145936847:	0.0%
Covered Lines:	380
Relevant Lines:	382

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💛 - Coveralls

[mondeja]

OK, I had not understood the decorator 😅 Thank you for the patch!