Add missing doc issues

[tomschr]

This covers issues #173.

I'm not sure where I can put #167 with the branch detection. It seems it is not very important at the moment.

Changes:

• docs/source/user/check-env.rst: A new "Checking your Environment" documenting the docbuild check files command (user guide).
• docs/source/glossary.rst: Add Pydantic definition into glossary
• docs/source/user/config.rst: Adjusted "Configuring the Tool" section and included placeholders (static & dynamic), key naming conventions, validating the configuration, using placeholders, and using multiple environment configuration. The latter was already there, I just gave it a title and moved it at the end of the file.

[github-actions]

Coverage Report

For commit d4fa162

Click to expand Coverage Report

  Name                                           Stmts   Miss Branch BrPart  Cover
  --------------------------------------------------------------------------------
+ src/docbuild/cli/cmd_check/process.py             60      0     22      1  98.8%
+ src/docbuild/utils/pidlock.py                     79      1     14      1  97.8%
+ src/docbuild/cli/cmd_validate/process.py         178      3     52      2  97.8%
+ src/docbuild/config/xml/stitch.py                 48      1     12      1  96.7%
+ src/docbuild/cli/callback.py                      35      0     10      2  95.6%
+ src/docbuild/cli/cmd_metadata/metaprocess.py     167     10     42      7  91.9%
- src/docbuild/cli/cmd_config/__init__.py            9      1      0      0  88.9%
- src/docbuild/cli/cmd_cli.py                       86     11      6      3  84.8%
- src/docbuild/cli/cmd_check/__init__.py            18      5      2      0  65.0%
- src/docbuild/cli/cmd_build/__init__.py            13      5      0      0  61.5%
- src/docbuild/cli/cmd_metadata/__init__.py         27     10      2      0  58.6%
- src/docbuild/cli/cmd_config/environment.py        11      6      2      0  38.5%
  --------------------------------------------------------------------------------
+ TOTAL                                           2744     53    638     17  97.8%
  
  47 files skipped due to complete coverage.

[sushant-suse]

Some minor suggestions for your consideration.